Fix typo (issue #19619).
Mention issue 20621 fix in the NEWS file for 3.3.5rc1.
This bug was of the major reasons for the release.
Issue #20759: Fix some typos in the mock docs.
Fix several C-API doc typos caught by tomo cocoa on docs@.

The signature and description of PyException_SetCause now use "cause"
rather than "ctx" to match the code.
Added tests for issue #20501.