[1.6.x] Corrected content_type parameter name in sitemap docs.

mimetype was deprecated in 1.5 per 11ec0253ab4cc926ab9e77619132cb398231ac33.

Backport of a7443c24a3 from master
[1.6.x] Fixed #23265 -- Used system-specific encoding in runserver

Thanks SpaceFox for the report.
Backport of 055d95fce066 from master.
[1.6.x] Fixed a grammar error in docs/ref/forms/api.txt.

Backport of bef5b9b7cd from master