[FSDP][6/N] Add note explaining idioms for `_FSDPState` traversal (#90959)
This adds a note to explain how to do traversal in the new code base. These traversal helper methods were introduced in [1/N], [3/N], and [5/N].
I am working on updating the traversal helpers to account for other composable APIs (e.g. `replicate`). The rule is that the traversal should not proceed into an incompatible API's tree. This will be needed for `fully_shard` to be above `replicate`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90959
Approved by: https://github.com/mrshenli
