Update on "[functorch] Add more details to the functorch install page"

Added some details about:
- `pip uninstall functorch` being helpful if there are problems
- `pip install functorch` still working for BC reasons.

Test Plan:
- wait for docs preview

[ghstack-poisoned]

functorch/docs/source/install.rst

@@ -10,7 +10,7 @@ If you're upgrading from an older version of functorch (functorch 0.1.x or 0.2.x
 then you may need to uninstall functorch first via ``pip uninstall functorch``.
 
 We've maintained backwards compatibility for ``pip install functorch``: this
-command works for PyTorch 1.13 and will continue to work for the forseeable future
+command works for PyTorch 1.13 and will continue to work for the foreseeable future
 until we do a proper deprecation. This is helpful if you're maintaining a library
 that supports multiple versions of PyTorch and/or functorch.