git-send-email.txt: mention less secure app access with Gmail

Google may have changed Gmail security and now less secure app access
needs to be explicitly enabled if two-factor authentication is not in
place, otherwise send-email fails with:

	5.7.8 Username and Password not accepted. Learn more at
	5.7.8  https://support.google.com/mail/?p=BadCredentials

Document steps required to make this work.

Signed-off-by: Vasyl Vavrychuk <vvavrychuk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
[dl: Clean up commit message and incorporate suggestions into patch.]
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-send-email.txt

@@ -494,10 +494,14 @@ edit ~/.gitconfig to specify your account settings:
 	smtpServerPort = 587
 ----
 
-If you have multifactor authentication setup on your gmail account, you will
+If you have multi-factor authentication set up on your Gmail account, you will
 need to 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 have multi-factor authentication set up on your Gmail account,
+you will need to allow less secure app access. Visit
+https://myaccount.google.com/lesssecureapps to enable it.
+
 Once your commits are ready to be sent to the mailing list, run the
 following commands: