docs: link OpenSSL's verify(1) manual page to know about -CAfile and -CApath options

docs: link OpenSSL's verify(1) manual page to know about -CAfile and -CApath options

The description of `--smtp-ssl-cert-path` in the git-send-email documentation
mentions consulting OpenSSL's verify(1) manual page for details about the
`-CAfile` and `-CApath` options. However, the way it was written was quite
confusing, and it didn't mention that OpenSSL's verify(1) is the manual page
to refer to.

Fix this by slightly rewording the description and also add a link to the
OpenSSL verify(1) manual page.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>