0cd6341cae
This PR is V2 of the[ PR created by @psarna.](https://github.com/scylladb/scylladb/pull/11560).
I have:
- copied the content.
- applied the suggestions left by @nyh.
- made minor improvements, such as replacing "Scylla" with "ScyllaDB", fixing punctuation, and fixing the RST syntax.
Fixes https://github.com/scylladb/scylladb/issues/11378
Closes #11984
* github.com:scylladb/scylladb:
doc: label user-defined functions as Experimental
doc: restore the note for the Count function (removed by mistatke)
doc: document user defined functions (UDFs)
(cherry picked from commit 7cbb0b98bb)