[T5 doc] Fix confusing documentation about `d_kv`
# What does this PR do?
Fixes #21641. The documentation mentioned that `d_kv` must be equal to `d_model//num_heads` while this does not hold in the code. The code states that `d_kv = inner_dim//num_heads`, but inner dim can be different from the d_model. 
_The documentation is not available anymore as the PR was closed or merged._