7cbb0b98bb
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)