[DOC] Add NEWS about Symbol GC

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45440 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

NEWS

@@ -23,6 +23,9 @@ with all sufficient information, see the ChangeLog file.
 * Symbol
   * New methods
     * Symbol.find(str) returns whether given string is defined as symbol or not.
+  * Improvements
+    * Most symbols in Ruby level are GC-able (generated by String#to_sym and
+      String#intern)
 
 === Core classes compatibility issues (excluding feature bug fixes)
 
@@ -89,3 +92,9 @@ with all sufficient information, see the ChangeLog file.
   However, weaknesses in hash distribution can no longer be masked
   by prime number-sized tables, so extensions may need to tweak
   hash functions to ensure good distribution.
+
+* rb_sym2str() added. This is almost same as `rb_id2str(SYM2ID(sym))`
+  but not pinning a dynamic symbol.
+
+* struct RSymbol added. This represents a dynamic symbol as object in
+  Ruby's heaps.