Better migration docs for get_query_set -> get_queryset

This way they won't need to fix things again when get_query_set disappears
Removed unused variable django.http.request.absolute_http_url_re
Simplified get_version() in django/__init__.py

Thanks Luke Plant for the report.
Moved release note for refs #21281 from "deprecation" to "backwards incompatible".
Fixed spelling in docs/ref/django-admin.txt.
Added deprecation of template dirs parameter to timeline.
Fixed #23543 -- Added docs on testing management command output.
Further fixes to the migration notes for get_query_set

This rename is very tricky for the case of subclasses which define
get_query_set and haven't been updated yet, which applies to all projects in
the form of RelatedManager from Django 1.5.
Fixed typo in django/utils/decorators.py comment.