Docs: Consistent use of "sign-up/sign up/signup" in wp-signup.php

[audrasjb]

As a follow-up to #41566, let's use a consistent writing for sign-up/signup in wp-signup.php docblocks.

I believe the correct use is:

1. "sign up" in a verbal phrase, which is used to refer to an action of signing up. Example: "Shows a form for a visitor to sign up for a new user account."
2. "sign-up" or "signup" when used as a noun or an adjective. Example: "Fires when the site or user sign-up process is complete."

As for now, the wp-signup.php file use the three forms. For consistency, I propose to use "sign up" on verbal phrases (which would be consistent with "log in", already used in the same file), and "sign-up" for the second case, which is the most current use (noun or adjective) in this file.

For reference: ​https://www.sporcle.com/blog/2018/03/sign-up-vs-signup-vs-sign-up-difference/

[audrasjb]

[silent 5.9 bugscrub]

The patch still applies cleanly against trunk, let's mark it ready for commit.

[peterwilsoncc]

In 51930:

Docs: Use sign-up & signup consistently in wp-signup.php.

In the docblocks throughout wp-signup.php use sign up for verbs and sign-up for nouns.

Props audrasjb, jeffpaul.
Fixes #54041. See #53399.