docs: fix broken llms.txt link and stale otherwise xref

- ai-coding-assistants.rst: use absolute https://datafusion.apache.org/python/llms.txt URL; the relative `llms.txt` resolved to /python/user-guide/llms.txt and 404'd because html_extra_path publishes the file at the site root.
- expressions.rst: drop the broken `:py:meth:~datafusion.expr.Expr.otherwise` xref (otherwise lives on CaseBuilder, not Expr) and spell the recommended replacement as `f.when(f.in_list(...), value).otherwise(default)`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/user-guide/ai-coding-assistants.rst

@@ -30,7 +30,7 @@ What is published
   DataFrame operations, expression building, SQL-to-DataFrame mappings,
   idiomatic patterns, and common pitfalls. Follows the
   `Agent Skills <https://agentskills.io>`_ open standard.
-- `llms.txt <llms.txt>`_ — an entry point for LLM-based tools following the
+- `llms.txt <https://datafusion.apache.org/python/llms.txt>`_ — an entry point for LLM-based tools following the
   `llmstxt.org <https://llmstxt.org>`_ convention. Categorized links to the
   skill, user guide, API reference, and examples.
 

docs/source/user-guide/common-operations/expressions.rst

@@ -234,9 +234,9 @@ This searched-CASE pattern is idiomatic for "attribute the measure to the
 matching side of a left join, otherwise contribute zero" — a shape that
 appears in TPC-H Q08 and similar market-share calculations.
 
-If a switched CASE has only two or three branches that test equality, an
-``in_list`` filter combined with :py:meth:`~datafusion.expr.Expr.otherwise`
-is often simpler than the full ``case`` builder.
+If a switched CASE only groups several equality matches into one bucket,
+``f.when(f.in_list(col(...), [...]), value).otherwise(default)`` is often
+simpler than the full ``case`` builder.
 
 Structs
 -------