doc: clean up deprecations.md doc

* This removes the --pending-deprecation label
  from the Buffer() constructor.  It was added originally,
  since it was just a Documentation only deprecation.
  It is now a full runtime deprecation,
  which by default will output what the  flag does anyway

doc/api/deprecations.md

@@ -133,7 +133,7 @@ changes:
     description: Documentation-only deprecation.
 -->
 
-Type: Runtime (supports [`--pending-deprecation`][])
+Type: Runtime
 
 The `Buffer()` function and `new Buffer()` constructor are deprecated due to
 API usability issues that can lead to accidental security issues.