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

Backport of 68d3db8a7b from master
[1.6.x] Converted seealso directives to use 4 space indendation.

Backport of affc04060f from master
[1.6.x] Explained why admindocs omits model methods with arguments.

Backport of 9e3aaa5a46 from master
[1.6.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
[1.6.x] Fixed #23737 -- Recommended the render() shortcut more strongly.

Thanks Aymeric Augustin for the report.

Backport of f85fcc75e3 from master
[1.6.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
[1.6.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