Cleanup pluggable_functions doc comment

Summary:
* Make the example actually compile.
* Avoid double negation.
* Fix function name.

Reviewed By: aristidisp

Differential Revision: D64725292

fbshipit-source-id: 0276a315ed86fb638806209483cc5dec589ae735

third-party/thrift/src/thrift/compiler/detail/pluggable_functions.h

@@ -24,16 +24,18 @@
 namespace apache::thrift::compiler::detail {
 
 /**
- * A simple reimplementation of thrift's PluggableFunction without using
- * dependencies banned from the compiler.
+ * A simple reimplementation of Thrift's PluggableFunction using only
+ * dependencies allowed in the compiler.
  *
- * Tag argument must have a static member function `defaultImpl` that is called
+ * Tag argument must have a static member function `default_impl` that is called
  * when the function is not set.
  *
  * Not thread-safe.
  *
- * Ex:
- *   struct MyTag { static void defaultImpl(int) {} };
+ * Example:
+ *   struct MyTag {
+ *     static void default_impl(int) {}
+ *   };
  *   pluggable_functions().set<MyTag>(+[](int){});
  *   pluggable_functions().call<MyTag>(42);
  */