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

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.

Review the property aliases in src/libtmux/pane.py, src/libtmux/window.py, src/libtmux/session.py, and src/libtmux/server.py, using the existing doctest_namespace fixtures as the usage context. For each alias, document its return type and value, then add a working doctest in the requested Examples and Returns sections. Done means all affected aliases have both sections and the doctests pass.