help: Improve documentation for Gitter import.

templates/zerver/help/import-from-gitter.md

@@ -92,8 +92,22 @@ to mark the appropriate users as administrators.
 - The [Gitter data export tool](https://github.com/minrk/archive-gitter)
   doesn't support exporting private gitter channels.
 
-- This tool doesn't yet support merging importing Gitter channels into
-  a single Zulip organization.
+- Gitter's export tool doesn't export email addresses; just GitHub
+  usernames.  The import tool will thus use [GitHub's generated
+  noreply email addresses][github-noreply] to compute the
+  GitHub-generated noreply email address associated with that GitHub
+  account, e.g.
+  `{github_user_id}+{github_username}@users.noreply.github.com` or
+  `{github_username}@users.noreply.github.com`.
+
+  Since one cannot receive email at those noreply email addresses,
+  imported users will need to use GitHub authentication to login to
+  Zulip and will be unable to receive email notifications until they
+  [change their Zulip email address](/help/change-your-email-address).
+
+- You can merge multiple Gitter channels into a single Zulip
+  organization using [this
+  tool](https://github.com/minrk/archive-gitter/pull/5).
 
 - This tool doesn't translate Gitter's markdown format into Zulip
   format markdown (there are a few corner cases where the syntax is
@@ -102,3 +116,4 @@ to mark the appropriate users as administrators.
   aren't translated into anything yet.
 
 [upgrade-zulip-from-git]: https://zulip.readthedocs.io/en/latest/production/upgrade-or-modify.html#upgrading-from-a-git-repository
+[github-noreply]: https://docs.github.com/en/github/setting-up-and-managing-your-github-user-account/setting-your-commit-email-address

zproject/backends.py

@@ -1453,10 +1453,12 @@ class GitHubAuthBackend(SocialAuthMixin, GithubOAuth2):
         # We disallow creation of new accounts with
         # @noreply.github.com/@users.noreply.github.com email
         # addresses, because structurally, we only want to allow email
-        # addresses that can receive emails, and those cannot. However,
-        # if an account with this address happens to already exist in the realm
-        # (which could happen e.g. as a result of data import from another chat tool),
-        # we will allow signing in to it.
+        # addresses that can receive emails, and those cannot.
+
+        # However, if an account with this address already exists in
+        # the realm (which could happen e.g. as a result of data
+        # import from another chat tool), we will allow signing in to
+        # it.
         email_objs = self.get_all_associated_email_objects(*args, **kwargs)
         return [
             email for email in email_objs