docs(docstrings): Add docstrings to dataclasses, fix citation syntax

why: Dataclasses without docstrings cause "Inline strong start-string
without end-string" warnings. Citation syntax [1]_ not resolving properly.

what:
- Add docstrings to ServerOptions, SessionOptions, WindowOptions,
  PaneOptions, Options dataclasses (constants.py)
- Simplify EnvironmentVarGuard docstring, convert citation to inline URL
  (environment.py)

Author: tony

src/libtmux/_internal/constants.py

@@ -26,6 +26,8 @@
 class ServerOptions(
     SkipDefaultFieldsReprMixin,
 ):
+    """Container for tmux server options."""
+
     backspace: str | None = field(default=None)
     buffer_limit: int | None = field(default=None)
     command_alias: SparseArray[str] = field(default_factory=SparseArray)
@@ -59,6 +61,8 @@ def __init__(self, **kwargs: object) -> None:
 class SessionOptions(
     SkipDefaultFieldsReprMixin,
 ):
+    """Container for tmux session options."""
+
     activity_action: t.Literal["any", "none", "current", "other"] | None = field(
         default=None,
     )
@@ -134,6 +138,8 @@ def __init__(self, **kwargs: object) -> None:
 class WindowOptions(
     SkipDefaultFieldsReprMixin,
 ):
+    """Container for tmux window options."""
+
     aggressive_resize: t.Literal["on", "off"] | None = field(default=None)
     automatic_rename: t.Literal["on", "off"] | None = field(default=None)
     automatic_rename_format: str | None = field(default=None)
@@ -197,6 +203,8 @@ def __init__(self, **kwargs: object) -> None:
 class PaneOptions(
     SkipDefaultFieldsReprMixin,
 ):
+    """Container for tmux pane options."""
+
     allow_passthrough: t.Literal["on", "off", "all"] | None = field(default=None)
     allow_rename: t.Literal["on", "off"] | None = field(default=None)
     alternate_screen: t.Literal["on", "off"] | None = field(default=None)
@@ -239,6 +247,8 @@ class Options(
     PaneOptions,
     SkipDefaultFieldsReprMixin,
 ):
+    """Container for all tmux options (server, session, window, and pane)."""
+
     def __init__(self, **kwargs: object) -> None:
         # Convert hyphenated keys to underscored attribute names and assign values
         # Remove asaterisk from inherited options

src/libtmux/test/environment.py

@@ -21,19 +21,13 @@
 class EnvironmentVarGuard:
     """Mock environmental variables safely.
 
-    Helps rotect the environment variable properly.  Can be used as context
+    Helps protect the environment variable properly. Can be used as context
     manager.
 
     Notes
     -----
     Vendorized to fix issue with Anaconda Python 2 not including test module,
-    see #121 [1]_
-
-    References
-    ----------
-    .. [1] Just installed, "ImportError: cannot import name test_support".
-       GitHub issue for tmuxp. https://github.com/tmux-python/tmuxp/issues/121.
-       Created October 12th, 2015. Accessed April 7th, 2018.
+    see `tmuxp#121 <https://github.com/tmux-python/tmuxp/issues/121>`_.
     """
 
     def __init__(self) -> None: