upload: Mention new difference between sanitize_name and slugify.
upload: Mention new difference between sanitize_name and slugify.

In Django 3.2 slugify strips trailing dashes and underscores:
https://github.com/django/django/commit/0382ecfe020b4c51b4c01e4e9a21892771e66941

sanitize_name doesn't so this difference should be documented like the
others.

andersk/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,rht/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,punchagan/zulip