[DOC] Improve some GC docs

Merged: https://github.com/ruby/ruby/pull/8219

gc.c

@@ -13813,7 +13813,7 @@ Init_GC(void)
         rb_hash_aset(gc_constants, ID2SYM(rb_intern("RB_BUG_INSTEAD_OF_RB_MEMERROR")), Qtrue);
     }
     OBJ_FREEZE(gc_constants);
-    /* internal constants */
+    /* Internal constants in the garbage collector. */
     rb_define_const(rb_mGC, "INTERNAL_CONSTANTS", gc_constants);
 
     rb_mProfiler = rb_define_module_under(rb_mGC, "Profiler");

gc.rb

@@ -1,6 +1,6 @@
 # for gc.c
 
-#  The GC module provides an interface to Ruby's mark and
+#  The \GC module provides an interface to Ruby's mark and
 #  sweep garbage collection mechanism.
 #
 #  Some of the underlying methods are also available via the ObjectSpace