runtime(doc): clarify term_cols allowed range in terminal.txt

runtime(doc): clarify term_cols allowed range in terminal.txt

Patch 9.0.1527 (related issue: vim/vim#12362) introduced range checking
for `term_cols` parameter of the various terminal related functions, but
did not update the documentation. This is the fix, hopefully I found
everything that mentions is. Also improve the style for `term_rows` to
match.

Code reference ( https://github.com/vim/vim/blob/master/src/job.c#L456 ):
	if (opt->jo_term_cols < 0 || opt->jo_term_cols > 1000)

closes: #19346

Signed-off-by: Kalin Kozhuharov <kalin@thinrope.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>