Document extended RuntimeTargetDelegate lifetime requirements

Summary:
Changelog: [Internal]

NOTE: This is a documentation-only diff.

Documents that the caller of `InstanceTarget::registerRuntime()` is required to keep the `RuntimeTargetDelegate&` valid longer than previously stated. Rather than merely outliving the `RuntimeTarget` object ( = past `unregisterRuntime()`), the delegate must also remain valid while any JS is being executed in the underlying runtime.

Proof that this requirement is already met by the existing integrations:
* In Bridgeless, `RuntimeTargetDelegate` is implemented by [`JSRuntime`](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/react/runtime/JSRuntimeFactory.h#L20). JS execution via the `RuntimeExecutor` happens [here](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp#L70), while a [strong reference to the `JSRuntime`](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp#L66) is in scope.
* In Bridge, `RuntimeTargetDelegate` is implemented by [`JSExecutor`](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/cxxreact/JSExecutor.h#L58). JS execution via the `RuntimeExecutor` happens [here](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/cxxreact/Instance.cpp#L253), while a (necessarily valid) non-owning [pointer to the `JSExecutor`](https://github.com/facebook/react-native/blob/9f85a249fa9e18eae745f103e390b2a8542e8ee9/packages/react-native/ReactCommon/cxxreact/Instance.cpp#L247) is in scope.

bypass-github-export-checks

Reviewed By: huntie

Differential Revision: D54493456

fbshipit-source-id: c8dc11b0696e20b5fe9f3e16bb7591be0b3b6157

packages/react-native/ReactCommon/jsinspector-modern/InstanceTarget.h

@@ -70,9 +70,22 @@ class InstanceTarget : public EnableExecutorFromThis<InstanceTarget> {
       FrontendChannel channel,
       SessionState& sessionState);
 
+  /**
+   * Registers a JS runtime with this InstanceTarget. \returns a reference to
+   * the created RuntimeTarget, which is owned by the \c InstanceTarget. All the
+   * requirements of \c RuntimeTarget::create must be met.
+   */
   RuntimeTarget& registerRuntime(
       RuntimeTargetDelegate& delegate,
       RuntimeExecutor executor);
+
+  /**
+   * Unregisters a JS runtime from this InstanceTarget. This destroys the \c
+   * RuntimeTarget, and it is no longer valid to use. Note that the \c
+   * RuntimeTargetDelegate& initially provided to \c registerRuntime may
+   * continue to be used as long as JavaScript execution continues in the
+   * runtime.
+   */
   void unregisterRuntime(RuntimeTarget& runtime);
 
  private:

packages/react-native/ReactCommon/jsinspector-modern/RuntimeTarget.h

@@ -77,15 +77,15 @@ class JSINSPECTOR_EXPORT RuntimeTarget
     : public EnableExecutorFromThis<RuntimeTarget> {
  public:
   /**
-   * Constructs a new RuntimeTarget. The caller must call setExecutor
-   * immediately afterwards.
    * \param executionContextDescription A description of the execution context
    * represented by this runtime. This is used for disambiguating the
    * source/destination of CDP messages when there are multiple runtimes
    * (concurrently or over the life of a Host).
    * \param delegate The object that will receive events from this target. The
-   * caller is responsible for
-   * ensuring that the delegate outlives this object.
+   * caller is responsible for ensuring that the delegate outlives this object
+   * AND that it remains valid for as long as the JS runtime is executing any
+   * code, even if the \c RuntimeTarget itself is destroyed. The delegate SHOULD
+   * be the object that owns the underlying jsi::Runtime, if any.
    * \param jsExecutor A RuntimeExecutor that can be used to schedule work on
    * the JS runtime's thread. The executor's queue should be empty when
    * RuntimeTarget is constructed (i.e. anything scheduled during the
@@ -127,9 +127,11 @@ class JSINSPECTOR_EXPORT RuntimeTarget
    * represented by this runtime. This is used for disambiguating the
    * source/destination of CDP messages when there are multiple runtimes
    * (concurrently or over the life of a Host).
-   * \param delegate The object that will receive events from this target.
-   * The caller is responsible for ensuring that the delegate outlives this
-   * object.
+   * \param delegate The object that will receive events from this target. The
+   * caller is responsible for ensuring that the delegate outlives this object
+   * AND that it remains valid for as long as the JS runtime is executing any
+   * code, even if the \c RuntimeTarget itself is destroyed. The delegate SHOULD
+   * be the object that owns the underlying jsi::Runtime, if any.
    * \param jsExecutor A RuntimeExecutor that can be used to schedule work on
    * the JS runtime's thread. The executor's queue should be empty when
    * RuntimeTarget is constructed (i.e. anything scheduled during the