Butter C++: clarify goals/intent in the comment

Summary:
Just clarifying the purpose of this library.

Changelog: [Internal]

Reviewed By: RSNara

Differential Revision: D33245053

fbshipit-source-id: b4325415774ca33b34b5ba53d222a19f88e2175f

ReactCommon/butter/butter.h

@@ -11,7 +11,7 @@ namespace facebook {
 namespace butter {
 
 /*
- * `Butter` is a trivial collection of basic tools borrowed from other low-level
+ * `Butter` is a minimal collection of basic tools borrowed from other low-level
  * general purpose libraries (like Folly, Abseil or Boost). The main goals of
  * Butter:
  * - Make the codebase more portable;
@@ -19,17 +19,26 @@ namespace butter {
  * Butter);
  * - Make relying on modern C++ patterns and tools in code simple and easy.
  * - Make executing experiments with different dependencies easier.
+ * - Limit reliance on third-party libraries eventually.
  *
+ * Target C++ Version
+ * ------------------
+ * Currently, Butter targets C++17.
+ *
+ * The Scope
+ * ---------
  * What should be part of Butter and what should not? Should I add some piece of
  * functionality in the Butter? Here is a quick checklist.
  *
+ * As of now, Butter is relatively permissive per the guidance below:
+ *
  * If one of the following is true, yes, go for it:
  * - If some feature is already in some future C++ standard (possibly in draft
  * stage) and it's already implemented in some 3rd party library.
  * - If some standardized feature of C++ is implemented in the standard not in
- * the most efficient way (because the standard enforces some tricky constraints
- * (like always-valid iterators) which nobody uses and should use), but you have
- * a library that does it right providing exact same interface.
+ * the most efficient way (because the standard enforces some tricky
+ * constraints, like always-valid iterators, which nobody uses and should use),
+ * but you have a library that does it right providing exact same interface.
  *
  * If one of the following is true, please do *NOT* do it (at least as part of
  * the library):
@@ -41,6 +50,11 @@ namespace butter {
  * faster than the standard one, so You want to use that in the code base. That
  * container does not have compatible API though (because it's a clear trade-off
  * with efficiency, of course).
+ *
+ * Note that eventually Butter will restrict the API collection to reduce
+ * reliance on non-standard C++ builtin libraries. That way, the API footprint
+ * stays small and is limited to just the essential APIs. This restriction is
+ * currently a work in progress.
  */
 
 /*