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.

This commit clarifies that app passwords are required and suggests using
OAuth2 if the user does not want to enable multi-factor authentication.

Signed-off-by: Junio C Hamano <[email protected]>

Author: AdityaGarg8

Documentation/git-send-email.adoc

@@ -524,9 +524,12 @@ edit `~/.gitconfig` to specify your account settings:
 	smtpServerPort = 587
 ----
 
+Gmail does not allow using your account password for `git send-email`.
 If you have multi-factor authentication set up on your Gmail account, you can
 generate an app-specific password for use with `git send-email`. Visit
 https://security.google.com/settings/security/apppasswords to create it.
+If you do not want to enable multi-factor authentication, you can use OAuth2.0
+authentication as described below.
 
 You can also use OAuth2.0 authentication with Gmail. `OAUTHBEARER` and
 `XOAUTH2` are common methods used for this type of authentication. Gmail