docs: mark try_avg and try_sum as natively supported

[andygrove]

Which issue does this PR close?

Part of #4098.

Rationale for this change

try_avg and try_sum were already fully implemented and tested natively: both parse to Spark's Average/Sum aggregates with EvalMode.TRY, which Comet's serde already plumbs through to the native accumulators (SumInteger, SumDecimal, AvgDecimal all handle Try mode, including overflow-to-NULL semantics). CometAggregateSuite already covers integer overflow, partial-overflow with GROUP BY, decimal overflow, null handling, and ANSI/non-ANSI combinations, and these tests pass with checkSparkAnswerAndOperator (confirming native execution and a match against Spark).

The only stale artifact was the expression support guide, which still listed both as not-yet-supported.

What changes are included in this PR?

Update docs/source/user-guide/latest/expressions.md to mark try_avg and try_sum as supported (✅), matching their non-try counterparts (avg notes that interval types fall back; sum has no caveat).

This change was scoped using the implement-comet-expression skill, which surfaced that the native implementation and test coverage already existed and that only the documentation needed updating.

How are these changes tested?

Existing try_avg/try_sum tests in CometAggregateSuite pass (10 tests, 0 failures), verifying native execution and Spark-matching results across overflow, group-by, decimal, and null cases.

[mbutrovich]

Thanks @andygrove!