Fix incorrect docs for user authentication functions

[ericlewis]

A few functions are related to logging in a user.

wp_signon() is the big one that extracts POST form data, authenticates the user, and logs the user in.

wp_authenticate() is responsible for authenticating the user: confirming the credentials actually match to an existing user. It does not "login" the user, just confirms credentials.

wp_set_auth_cookie() is where the rubber meets the road, and login cookies are set appropriately.

get_currentuserinfo() will consider a user logged in by checking that the cookie is set.

Some of these functions have misleading docs, I'd like to set the record straight.

[ericlewis]

attachment:35387.2.diff​ drops some tests I had leftover from a previous diff.

[ericlewis]

In 36245:

Users: Fix incorrect docs for user authentication and login functions.

Fixes #35387

[ericlewis]

In 36246:

Login is not a verb, change "login" to "log in".

See #35387