Strengthen docstrings in metrics.py:   - Make clearer distinction between description (regular text), code literals     (enclosed in double backticks), and math expressions.   - Also make minor changes to content of a few docstrings.
Strengthen docstrings in metrics.py:
  - Make clearer distinction between description (regular text), code literals
    (enclosed in double backticks), and math expressions.
  - Also make minor changes to content of a few docstrings.

Trying this out in a small change first. Once the results look right (in
readthedocs), will apply the same systematically across the code base.

PiperOrigin-RevId: 346836009

google/trax,google/trax