docs(API): Document internal constants

Author: tony

docs/internals/constants.md

@@ -0,0 +1,15 @@
+# Internal Constants - `libtmux._internal.constants`
+
+:::{warning}
+Be careful with these! These constants are private, internal as they're **not** covered by version policies. They can break or be removed between minor versions!
+
+If you need a data structure here made public or stabilized please [file an issue](https://github.com/tmux-python/libtmux/issues).
+:::
+
+```{eval-rst}
+.. automodule:: libtmux._internal.constants
+   :members:
+   :undoc-members:
+   :inherited-members:
+   :show-inheritance:
+```

docs/internals/index.md

@@ -11,6 +11,7 @@ If you need an internal API stabilized please [file an issue](https://github.com
 ```{toctree}
 dataclasses
 query_list
+constants
 ```
 
 ## Environmental variables