[duckdb/#15939] - Add the `TRY` expression needs documentation

[duckdblabs-bot]

See duckdb/duckdb#15939

[Tishj]

I would hold off on this, there is another limitation that needs to be added

TRY(...) will make sure the errors caused by the input rows in the child (scalar) expression result in NULL for that row / those rows, instead of causing the query to end in error.

This is not supported for unstable functions like random(), or scalar subqueries, or aggregates*

* this one is new

[szarnyasg]

Added a milestone for v1.3. Fish test cases for inspiration:

https://github.com/duckdb/duckdb/pull/15939/files#diff-cfefc5dfd5806e645e1b8fdc05738d2ef1e5142ec15f7e8d0205ade34960bcfb