symbol.c: remove `id2name` reference from documents.

src/kernel.c

@@ -510,7 +510,7 @@ mrb_method_missing(mrb_state *mrb, mrb_sym name, mrb_value self, mrb_value args)
  *         # ...
  *       end
  *       def method_missing(methId)
- *         str = methId.id2name
+ *         str = methId.to_s
  *         romanToInt(str)
  *       end
  *     end

src/string.c

@@ -1873,7 +1873,7 @@ mrb_str_init(mrb_state *mrb, mrb_value self)
  *     str.to_sym   => symbol
  *
  *  Returns the <code>Symbol</code> corresponding to <i>str</i>, creating the
- *  symbol if it did not previously exist. See <code>Symbol#id2name</code>.
+ *  symbol if it did not previously exist.
  *
  *     "Koala".intern         #=> :Koala
  *     s = 'cat'.to_sym       #=> :cat

src/symbol.c

@@ -389,12 +389,11 @@ mrb_init_symtbl(mrb_state *mrb)
 /* 15.2.11.3.3  */
 /*
  *  call-seq:
- *     sym.id2name   -> string
- *     sym.to_s      -> string
+  *     sym.to_s      -> string
  *
  *  Returns the name or string corresponding to <i>sym</i>.
  *
- *     :fred.id2name   #=> "fred"
+ *     :fred.to_s   #=> "fred"
  */
 static mrb_value
 sym_to_s(mrb_state *mrb, mrb_value sym)