Improve docs for 'eval-when'.

Improve docs for 'eval-when'.

* doc/ref/api-macros.texi (Eval When): Explain in detail what each
  condition means, including 'expand' which was previously undocumented.
  Change the example to use (expand load eval) and recommend that set of
  conditions, instead of (compile load eval) which was previously
  recommended and shown in the example.