docs: correct window function support status and limitations

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

The documentation for window functions was out of date and contradicted itself. The compatibility guide (compatibility/operators.md) and the contributor roadmap still stated that native window execution is "disabled by default and known to be incorrect" and "should not be used in production", citing the epic #2721. That epic, along with the other referenced issues (#2705, #2841, #4007), is now closed.

In reality, WindowExec runs natively and spark.comet.exec.window.enabled defaults to true. A broad set of window functions is accelerated, and any shape Comet does not handle falls back to Spark rather than producing an incorrect result. The expressions.md and operators.md pages already reflected this, so the guide and roadmap were the stale pages that disagreed with them.

What changes are included in this PR?

• Rewrite the "Window Functions" section of compatibility/operators.md to describe the current behavior: the ranking, value, and aggregate window functions that are accelerated, the supported frame units and bounds, and the specific cases that fall back to Spark.
• Update the WindowExec note in operators.md to drop the closed-issue reference and point to the window compatibility section.
• Tighten the window_funcs prose in expressions.md to note that statistical aggregates run natively as plain aggregations but fall back as window functions, and to link to the compatibility guide.
• Rewrite the "Window Expressions" roadmap section, which cited only closed issues, to describe what is accelerated today and the genuinely remaining gaps.
• State the unsupported aggregate input types positively (the Spark types Comet does not accept) rather than by exclusion. These input-type limitations are shared with the batch aggregate path, so the wording notes they apply in both contexts rather than being window-specific.
• Link every remaining gap to a tracking issue. Existing issues: statistical aggregates as window functions (Extend windowed aggregate allow-list (collect_set, collect_list, and other natively-supported aggregates) #4766), non-literal lag/lead default (Support non-Literal default value for LAG/LEAD #4268), decimal sliding-frame sum/avg ([Bug] SUM(decimal) over a sliding window frame returns wrapped out-of-range value on overflow instead of NULL #4729, closed by the intentional fallback). New issues filed for the previously untracked gaps: RANGE frame with an explicit date/decimal offset (Support RANGE window frames with an explicit offset on DATE or DECIMAL ORDER BY #4834), first_value/last_value on a RANGE frame with a literal offset (Support FIRST_VALUE/LAST_VALUE on a RANGE window frame with a literal offset #4835), GROUPS frame units (Support GROUPS window frame units #4836), and WindowGroupLimitExec (Support WindowGroupLimitExec (window-based limit pushdown) #4837).

How are these changes tested?

Documentation-only change. Formatting was verified with prettier --check.

[andygrove]

@comphead could you review this for accuracy?

[comphead]

Thanks @andygrove
you mentioned #4766 but we also need to mentioned collect_list, collect_set is in progress

[andygrove]

Thanks @andygrove you mentioned #4766 but we also need to mentioned collect_list, collect_set is in progress

Thanks. I added this.