docs: improve send-email documentation

OAuth2.0 is a new authentication method that is being used by many email
providers, including Outlook and Gmail. Recently, the Authen::SASL perl
module has been updated to support OAuth2.0 authentication, thus making
the git-send-email script be able to use this authentication method as
well. So lets improve the documentation to reflect this change.

I also had a hard time finding a reliable OAuth2.0 access token
generator for Outlook and Gmail. So I added a link to the such
generators which I developed myself after seaching through lots of code
and API documentation to make things easier for others.

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

Documentation/git-send-email.adoc

@@ -496,12 +496,12 @@ include::includes/cmd-config-section-all.adoc[]
 
 include::config/sendemail.adoc[]
 
-EXAMPLES
---------
-Use gmail as the smtp server
+EXAMPLES OF SMTP SERVERS
+------------------------
+Use Gmail as the SMTP Server
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-To use 'git send-email' to send your patches through the GMail SMTP server,
-edit ~/.gitconfig to specify your account settings:
+To use `git send-email` to send your patches through the Gmail SMTP server,
+edit `~/.gitconfig` to specify your account settings:
 
 ----
 [sendemail]
@@ -515,6 +515,41 @@ 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.
 
+You can also use OAuth2.0 authentication with Gmail. `OAUTHBEARER` and
+`XOAUTH2` are common methods used for this type of authentication. Gmail
+supports both of them. As an example, if you want to use `OAUTHBEARER`, edit
+your `~/.gitconfig` file and add `smtpAuth = OAUTHBEARER` to your account
+settings:
+
+----
+[sendemail]
+	smtpEncryption = tls
+	smtpServer = smtp.gmail.com
+	smtpUser = yourname@gmail.com
+	smtpServerPort = 587
+	smtpAuth = OAUTHBEARER
+----
+
+Use Microsoft Outlook as the SMTP Server
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Unlike Gmail, Microsoft Outlook no longer supports app-specific passwords.
+Therefore, OAuth2.0 authentication must be used for Outlook. Also, it only
+supports `XOAUTH2` authentication method.
+
+Edit `~/.gitconfig` to specify your account settings for Outlook and use its
+SMTP server with `git send-email`:
+
+----
+[sendemail]
+	smtpEncryption = tls
+	smtpServer = smtp.office365.com
+	smtpUser = yourname@outlook.com
+	smtpServerPort = 587
+	smtpAuth = XOAUTH2
+----
+
+SENDING PATCHES
+---------------
 Once your commits are ready to be sent to the mailing list, run the
 following commands:
 
@@ -523,9 +558,25 @@ following commands:
 	$ git send-email outgoing/*
 
 The first time you run it, you will be prompted for your credentials.  Enter the
-app-specific or your regular password as appropriate.  If you have credential
-helper configured (see linkgit:git-credential[1]), the password will be saved in
-the credential store so you won't have to type it the next time.
+app-specific or your regular password as appropriate.
+
+If you have a credential helper configured (see linkgit:git-credential[1]), the
+password will be saved in the credential store so you won't have to type it the
+next time.
+
+If you are using OAuth2.0 authentication, you need to use an access token in
+place of a password when prompted. Various OAuth2.0 token generators are
+available online. Community maintained credential helpers for Gmail and Outlook
+are also available:
+
+	- https://github.com/AdityaGarg8/git-credential-email[git-credential-gmail]
+	  (cross platform, dedicated helper for authenticating Gmail accounts)
+
+	- https://github.com/AdityaGarg8/git-credential-email[git-credential-outlook]
+	  (cross platform, dedicated helper for authenticating Microsoft Outlook accounts)
+
+You can also see linkgit:gitcredentials[7] for more OAuth based authentication
+helpers.
 
 Note: the following core Perl modules that may be installed with your
 distribution of Perl are required: