[1.7.x] Removed confusing paragraph from the docs.

This snippet wasn't particularly helpful as `add_error()` is described
in greater details in the following section.

Thanks Claude Paroz for the report.

Backport of f1f0c4f16b from master
[1.7.x] Removed redundant link in 1.7 release notes.

Backport of 9f4c718b2f from master
[1.7.x] Fixed #23770 -- Changed serialization strategy for floats with respect to NaN and Inf

Thanks to w0rp for the report

Backport of c7c098cf97fbb1416f302c76799058ad63a5f7aa from master.
Mysql db check should handle None db_type returns. Backport from https://github.com/django/django/commit/e9103402c0fa873aea58a6a11dba510cd308cb84#diff-14
Release note for db_type mysql fix
Add regression tests for ticket 23761