refactor: split the commit rule from the planner-rebinding note

The rule that everything after the planner install must be infallible
was tacked onto the end of a comment arguing something else -- why the
rebind is guarded on a call that installs nothing. Two unrelated
arguments in one block, with the more important of the two reading as
a footnote to the other. Give it its own block.

Its pointer at the reasoning was a `:ref:` role, which renders nowhere
from a `#` comment and leaves a reader who follows it holding a label
with no way to resolve it. Name the file and the heading instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: timsaucer

python/datafusion/context.py

@@ -2245,11 +2245,14 @@ def with_extensions(
         # it already holds, so the rebuild is unobservable except in the one case
         # where it does harm: a planner sitting on some *other* handle's codecs
         # gets dragged onto this handle's, silently undoing that install.
-        # Everything below this line must be infallible. A registration whose
-        # commit can fail belongs above, split into an import step that returns
-        # a resolved object and an insert step that cannot raise -- there is one
-        # session here, shared with the receiver, so a failure part-way through
-        # has nothing to roll back to. See :ref:`ffi_internals_commit_order`.
+
+        # Commit. Everything below this line must be infallible. A registration
+        # whose commit can fail belongs above, split into an import step that
+        # returns a resolved object and an insert step that cannot raise --
+        # there is one session here, shared with the receiver, so a failure
+        # part-way through has nothing to roll back to. The reasoning is in
+        # docs/source/contributor-guide/ffi-internals.md, under "Why
+        # `with_extensions` commits last".
         if planner is not None or logical_codecs or physical_codecs:
             new.ctx._install_extension_planner(planner)
         for register, functions in resolved: