docs: add a paragraph explaining the `sendmailCmd` option of sendemail

`sendmailCmd` is a configuration option in `git-send-email` that allows
users to send emails using an external application that supports
sendmail-like commands. This ability has been very useful to support
proprietary email APIs without modifying the `git-send-email` codebase.
It is also useful for users who prefer to use another SMTP client
instead of the SMTP perl library used by `git-send-email`.
This commit adds a paragraph to the documentation explaining this
option.

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

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