feature #5430 Pr/5085 (sjagr, javiereguiluz)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes #5430).

Discussion
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Pr/5085

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
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bafbd87 Fixed some minor syntax issues.
eb16061 Reflect links correctly in gmail.rst
60703ba Revisions to the gmail.rst doc
0320b5e Update gmail.rst

Author: weaverryan

cookbook/email/gmail.rst

@@ -75,3 +75,13 @@ You're done!
 
     The ``gmail`` transport is simply a shortcut that uses the ``smtp`` transport
     and sets ``encryption``, ``auth_mode`` and ``host`` to work with Gmail.
+
+.. note::
+
+    Depending on your Gmail account settings, you may get authentication errors
+    within your app. If your Gmail account uses 2-Step-Verification, you should
+    `generate an App password`_ to use for your ``mailer_password`` parameter.
+    You should also ensure that you `allow less secure apps to access your Gmail account`_.
+
+.. _`generate an App password`: https://support.google.com/accounts/answer/185833
+.. _`allow less secure apps to access your Gmail account`: https://support.google.com/accounts/answer/6010255