SQL: document the limitations around parsing

[astefan]

When #33902 has been merged, a limit has been added in the parser to prevent lengthy complex queries to generate a StackOverflowError. This means that not all SQL queries will work in ES SQL.
This issue is proposing to document this limit in the Limitations page.

[elasticmachine]

Pinging @elastic/es-search

[matriv]

This is closed as part of #41835.
@astefan any objections?