Update comments to operator_check

Update comments to operator_check

Hello,
I happen to read the code and find the comments to operator_check are
incorrect.  This patch is to fix the comments per my understanding.
The comments and field operator_check was added by this patch

  https://sourceware.org/ml/gdb-patches/2010-04/msg00556.html

but the inconsistency between code and comments wasn't pointed out during
the review.

gdb:

2014-07-30  Yao Qi  <yao@codesourcery.com>

	* parser-defs.h (struct exp_descriptor) <operator_check>: Update
	comments.
	* parse.c (exp_iterate): Update comments.