[ruby/zlib] Document that gets can return nil when eof? returns false

This behavior differs from File#gets.

Fixes [Bug #13904]

https://github.com/ruby/zlib/commit/448e41efdd

ext/zlib/zlib.c

@@ -4229,6 +4229,8 @@ gzreader_gets(int argc, VALUE *argv, VALUE obj)
  * Document-method: Zlib::GzipReader#gets
  *
  * See Zlib::GzipReader documentation for a description.
+ * However, note that this method can return +nil+ even if
+ * #eof? returns false, unlike the behavior of File#gets.
  */
 static VALUE
 rb_gzreader_gets(int argc, VALUE *argv, VALUE obj)