doc: add distinct for aggregate functions

Summarize the changes

• doc: add distinct for aggregate functions

Ensure that

• New code is covered by unit and integration tests.
• Related issues would be automatically closed with gitlab's closing pattern (Closes #1, #2).
• [] Public modules are documented (check the rendered version with cargo doc --open`).
• (if PEST grammar is changed) EBNF grammar reflects these changes (check the result with railroad diagram generator.

Next steps

• Cherry-pick to: none / 24.2 / 24.3
• Update sbroad submodule in picodata/picodata.
• (if EBNF grammar is changed) create a follow-up issue in picodata/docs.