DEP: linalg: deprecate expm2 and expm3

The Pade approximant provided by expm is a superior alternative, so
providing two worse methods just clutters the documentation.
DOC: linalg: explain the Taylor order used in expm3

Better to fix this in the documentation than to change the definition.

Fixes gh-2212
DOC: update release notes on deprecations
Merge pull request #2738 from pv/deprecate-expm23

 DEP: linalg: deprecate expm2 and expm3