formats(docs[PANE_FORMATS]): clarify lists are reference-only, not runtime

why: PANE_FORMATS and sibling constants are never imported at runtime;
     the live mechanism is neo.Obj.__dataclass_fields__ + get_output_format().
     Without this docstring, the pane_title entry uncommented in this PR
     looks like dead code that should be removed.
what:
- Expand module docstring to state constants are reference documentation
- Point future maintainers to neo.Obj and get_output_format()

Author: tony

src/libtmux/formats.py

@@ -3,6 +3,12 @@
 libtmux.formats
 ~~~~~~~~~~~~~~~
 
+These lists are **reference documentation only** — they are not imported
+or consumed at runtime. The active format mechanism is
+:class:`libtmux.neo.Obj`, whose dataclass fields are introspected by
+:func:`libtmux.neo.get_output_format` to build tmux format strings
+dynamically.
+
 For reference: https://github.com/tmux/tmux/blob/master/format.c
 
 """