Refined the warning message for numbered-parameter like variables

[Bug #16438]

NEWS

@@ -127,7 +127,7 @@ sufficient information, see the ChangeLog file or Redmine
 
   You can still define a local variable named _1 and so on, and that is honored when present, but renders warning.
 
-    _1 = 0            #=> warning: `_1' is used as numbered parameter
+    _1 = 0            #=> warning: `_1' is reserved for numbered parameter; consider another name
     [1].each { p _1 } # prints 0 instead of 1
 
 ==== proc/lambda without block is deprecated

parse.y

@@ -11836,7 +11836,7 @@ static void
 numparam_name(struct parser_params *p, ID id)
 {
     if (!NUMPARAM_ID_P(id)) return;
-    rb_warn1("`_%d' is reserved as numbered parameter",
+    rb_warn1("`_%d' is reserved for numbered parameter; consider another name",
 	     WARN_I(NUMPARAM_ID_TO_IDX(id)));
 }
 

test/ruby/test_syntax.rb

@@ -1437,7 +1437,7 @@ eom
     assert_syntax_error('-> {_1; -> {_2}}', /numbered parameter is already used/)
     assert_syntax_error('-> {-> {_1}; _2}', /numbered parameter is already used/)
     assert_syntax_error('proc {_1; _1 = nil}', /Can't assign to numbered parameter _1/)
-    mesg = proc {|n| /`_#{n}' is reserved as numbered parameter/}
+    mesg = proc {|n| /`_#{n}' is reserved for numbered parameter/}
     assert_warn(mesg[1]) {eval('proc {_1 = nil}')}
     assert_warn(mesg[2]) {eval('_2=1')}
     assert_warn(mesg[3]) {eval('proc {|_3|}')}