TF train_step docstring
# What does this PR do?

Small PR that elaborates on the beautiful black magic behind our TF `train_step`.
_The documentation is not available anymore as the PR was closed or merged._<|||||>LGTM! Maybe wait until tomorrow to merge because people are off, though, and I want to confirm a verbose docstring like this is okay.<|||||>Tagging @sgugger to check @Rocketknight1's question above ☝️ <|||||>Also, don't forget `make style`!<|||||>> Can we break the docstring in several paragraphs? For instance at the "However" and then "This is" (but whatever you think is best works too).

Done 👍 