docs(skill): record DataFrame.to_string as not needing exposure

str(df) is the Pythonic way to get a string and already uses __repr__
and the configurable formatter, so check-upstream should not flag the
upstream to_string as a gap.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: timsaucer

.ai/skills/check-upstream/SKILL.md

@@ -146,6 +146,7 @@ The user may specify an area via `$ARGUMENTS`. If no area is specified or "all"
 - `show_limit` — already covered by `DataFrame.show()`, which provides the same functionality with a simpler API
 - `with_param_values` — already covered by the `param_values` argument on `SessionContext.sql()`, which accomplishes the same thing more robustly
 - `union_by_name_distinct` — already covered by `DataFrame.union_by_name(distinct=True)`, which provides a more Pythonic API
+- `to_string` — `str(df)` is the Pythonic way to get a string and already goes through `__repr__` and the configurable formatter. A separate `to_string()` would either duplicate `str(df)` or render every row through a different path (session `datafusion.format.*` options, no formatter), giving a third text rendering alongside `repr` and `show()`
 
 **How to check:**
 1. Fetch the upstream DataFrame documentation page listing all methods