add some errhint for regexp* functions.

jian he <jian.universality@gmail.com> writes:

Some of the regexp* function error messages seem not very helpful, I think.
like:
ereport(ERROR,
errcode(ERRCODE_INVALID_PARAMETER_VALUE),
errmsg("invalid value for parameter \"%s\": %d",
"start", start));

so I added a errhint, errhint would be better than refactoring existing errmsg.
for the above example, I've add:

+ errhint("\"%s\" must be an integer value greater than zero", "start")));

There are probably hundreds of other places where the ereport
doesn't provide such detail, and I don't see that this adds
much. If you don't understand why a negative value isn't OK,
you still need to go RTFM.

(This message doesn't follow our style guidelines, either.)

regards, tom lane