formats(fix[docs]): scope "reference-only" claim to *_FORMATS lists

why: The module docstring added in 4f2ae6e said "not imported or consumed
     at runtime" — but FORMAT_SEPARATOR is imported by neo, pane, and
     session at runtime. The blanket claim was inaccurate.
what:
- Scope the reference-only note to *_FORMATS lists explicitly
- Add a sentence naming FORMAT_SEPARATOR as the runtime-used constant

Author: tony

src/libtmux/formats.py

@@ -3,11 +3,13 @@
 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.
+:data:`FORMAT_SEPARATOR` is used at runtime by ``neo``, ``pane``, and
+``session``.  The ``*_FORMATS`` lists (``SESSION_FORMATS``,
+``WINDOW_FORMATS``, ``PANE_FORMATS``, etc.) 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