TF: correct TFBart embeddings weights name when load_weight_prefix is passed
# What does this PR do?

Follow up to #18939 -- the embeddings weights were not being named correctly when `load_weight_prefix` was being passed. A few comments were also added so that our future selves remember how TF sets the names to their variables.

This was the cause of the TFRag test failures, as the loaded TFBart (`self.rag.generator`) was not getting its embedding weights.
cc @ydshieh -- this PR fixes the TFRag test we've seen in the scheduled CI run<|||||>_The documentation is not available anymore as the PR was closed or merged._