Some minor polishing of the release docs for 2.19.

1. Fix indentation.  The indentation of the first three bullet points in the markdown sources did not match the indentation of the fourth and fifth bullet points, nor of the bullet points further below.

2. Wrap some long lines in the markdown sources, in particular where there were some
lines wrapped but others not wrapped in the same bullet point list.

3. Use "Python API" rather than "Interpreter" as the subheading for changes
affecting the `tf.lite.Interpreter` Python class, for consistency with the earlier
heading "C++ API" in the same bullet point list.

PiperOrigin-RevId: 729490607