Further strengthening of attention docstrings.
Further strengthening of attention docstrings.

Also rename DotProductAttentionFn --> _per_head_attention. Benefits:
  - Makes the function private (no clients detected outside attention.py).
  - Aligns with general naming pattern for functions in codebase (camel-case
    for functions that return layers; lower case with underscores otherwise).

PiperOrigin-RevId: 358943309

google/trax,google/trax