[1.7.x] Fixed #23725 -- Substituted AUTH_USER_MODEL for User in docs.

Backport of 68d3db8a7b from master
[1.7.x] Fixed #23575 -- Added a code example for custom AdminSite.

Backport of 4a9b4a23bd from master
[1.7.x] Fixed #23558 -- documented slugify limitations

Backport of 03467368db from master
[1.7.x] Updated FIRST_DAY_OF_WEEK for Ukrainian to Monday.

Backport of eab3dc195e from master
[1.7.x] Fixed #23152 -- Added support for transactional Spatialite metadata initialization.

Thanks Doug Goldstein for the initial patch.

Backport of 57e40551e4 from master
[1.7.x] Converted seealso directives to use 4 space indendation.

Backport of affc04060f from master
[1.7.x] Removed extra period.

Backport of 8f79c15 from master
[1.7.x] Explained why admindocs omits model methods with arguments.

Backport of 9e3aaa5a46 from master
[1.7.x] Fixed #23734 -- Replaced striptags in template filter overview since it has security implications.

Thanks Aymeric Augustin for the suggestion.

Backport of 08c5887f13 from master
[1.7.x] Fixed #23737 -- Recommended the render() shortcut more strongly.

Thanks Aymeric Augustin for the report.

Backport of f85fcc75e3 from master
[1.7.x] Fixed #23736 -- Corrected the description of the value that silent_variable_failure uses.

Thanks Aymeric Augustin for the report.

Backport of e7cb4f2fba from master
[1.7.x] Fixed #23735 -- Clarified that admindocs must be enabled to view template tag libraries in admin.

Thanks Aymeric Augustin for the report.

Backport of 19242c675f from master