docs: document ReusedExchange caveat and both CometSparkToColumnar names in operator-count exclusions

[andygrove]

Which issue does this PR close?

Docs follow-up to review feedback on #5206, which was deferred to keep that PR's CI
scope small.

Rationale for this change

#5206 added a section to understanding-comet-plans.md listing the nodes excluded from
the operator counts in the extended explain summary. Review raised two gaps in that
list.

What changes are included in this PR?

1. Both rendered names of CometSparkToColumnarExec are listed. nodeName returns
   CometSparkColumnarToColumnar when child.supportsColumnar is true and
   CometSparkRowToColumnar otherwise, but the transition-nodes bullet only named the
   latter. Both are already excluded today, because generateTreeString matches on the
   Scala type rather than the rendered name, so a reader who sees
   CometSparkColumnarToColumnar in their own explain output could not tell from the
   list that it was covered. The bullet now names both and says they are one operator
   under two names. This matches the Columnar/Row Transitions table further down, which
   already documents both names.

2. ReusedExchange is now covered, with its caveat. It is matched in the same
   ignore arm as AdaptiveSparkPlanExec, InputAdapter, WholeStageCodegenExec,
   QueryStageExec, and AQEShuffleReadExec, but was not mentioned anywhere in the
   section. It also does not behave like those wrappers: getActualPlan unwraps it to
   its child, so the reused subtree is counted once per reference rather than once for
   the whole plan. Since the section exists to explain what is excluded and why, the new
   bullet states that the node itself is skipped, spells out the per-reference
   double-counting with a concrete example, and points at item 3 of Extended explain operator stats miscount reuse wrappers and CometSubqueryBroadcast #5203 as the
   tracking issue.

No prose was removed, and no non-docs files are touched.

How are these changes tested?

Docs-only. npx prettier "docs/source/user-guide/latest/understanding-comet-plans.md" --check passes. No new in-page anchor links were added: MyST slugifies the
Columnar/Row Transitions heading with the slash dropped, so a natural-looking
#columnar-row-transitions link would have been broken, and the two node names are
explained inline in the bullet instead.

[mbutrovich]

Thanks @andygrove!