for-each-ref: reword the documentation for '--start-after'

for-each-ref: reword the documentation for '--start-after'

The documentation for '--start-after' states that the flag cannot be
used with general pattern matching. This is a bit vague, since there is
no clear understanding about what 'general' means here. Rewrite the
sentence to be more specific.

While here, fix a typo in the 'OPT_STRING'.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>