[DX] Make user_save() more backwards-compatible

[klonos]

The signature of the user_save() function has changed in Backdrop:

D7

function user_save($account, $edit = array(), $category = 'account') {
  ...
}

Backdrop:

function user_save($account) {
  return $account->save();
}

I think that we should:

• mention https://api.backdropcms.org/change-records/user-entity-now-classed-object in the function docblock, along with this:

  Drupal 8 removed user_save() entirely; in Backdrop user_save() remains as a wrapper around $user->save().

• re-introduce the $edit and $category parameters, and have user_save() throw watchdog warnings when these are being passed, linking to https://api.backdropcms.org/change-records/user-entity-now-classed-object

[klonos]

I have put together a PR, which is WIP: backdrop/backdrop#2787

The idea is to have most of the docblock changes ready for review/feedback. I think that we need some sort of an equivalent of watchdog_deprecated_function() for deprecated function parameters. Something like watchdog_deprecated_function_argument() ...but that'd be a separate issue.