docs: make the purpose of using app password for Gmail more clear in send-email

The current example for Gmail suggests using app passwords for
send-email if user has multi-factor authentication set up for their
account. However, it does not clarify that the user cannot use their
normal password in case they do not have multi-factor authentication
enabled. Most likely the example was written in the days when Google
allowed using normal passwords without multi-factor authentication.

Clarify that regular passwords do not work for Gmail and app-passwords
are the only way for basic authentication. Also encourage users to use
OAuth2.0 as a more secure alternative.

While at it, also prefer using the word "mechanism" over "method" for
`OAUTHBEARER` and `XOAUTH2` since that is what official docs use.

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

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