io.c: [DOC] IO#gets [skip ci]

* io.c (rb_io_gets_m): [DOC] add example using limit argument.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@52489 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

io.c

@@ -3292,6 +3292,8 @@ rb_io_gets(VALUE io)
  *     File.new("testfile").gets   #=> "This is line one\n"
  *     $_                          #=> "This is line one\n"
  *
+ *     File.new("testfile").gets(4)#=> "This"
+ *
  *  If IO contains multibyte characters byte then <code>gets(1)</code>
  *  returns character entirely:
  *