[1.6.x] Fixed #21787 -- regression in MTI .exclude() queries

Backpatch of 78a2617703bc1dada409f126db5c3db214913ff4 from master.
[1.6.x] Fixed #21824 -- Added reference to LTS in docs/internals/security.txt

Backport of 4d8209431d from master
[1.6.x] Fixed #21869 -- Fixed docs building with Sphinx 1.2.1.

Thanks tragiclifestories for the report.

Backport of e1d18b9d2e from master