docs(docstrings): Add Examples/Returns sections to property aliases

[tony]

Summary

Many property aliases across the core ORM classes lack Examples and Returns sections in their docstrings. Adding these would improve API discoverability and provide inline usage documentation.

Affected files

• src/libtmux/pane.py
• src/libtmux/window.py
• src/libtmux/session.py
• src/libtmux/server.py

What to add

For each property alias (e.g. session_name, window_id, pane_id, etc.):

1. Returns section — document the return type and what the value represents
2. Examples section — add a working doctest showing usage with the doctest_namespace fixtures

Example of the desired pattern:

@property
def pane_id(self) -> str | None:
    """Unique pane ID.

    Returns
    -------
    str or None
        Pane ID in the format ``%N``, e.g. ``%0``.

    Examples
    --------
    >>> pane.pane_id  # doctest: +ELLIPSIS
    '%...'
    """

Context

Found during code review of PR #636. This is a documentation-only enhancement with no functional changes.