Add action that fires when inviting a user

[ebinnion]

I am currently looking into adding WordPress.com notifications that fire when a user is invited to a site.

While it is possible to use the add_option hook and then check that $option is of the form new_user_, I don't believe it is optimal.

I suggest that we add a new action that is fired during the invite user process.

[ebinnion]

Adds invite_user action

[dd32]

In the proposed patch, I think the current_user parameter can be skipped.. Any hooked functions can simply grab the logged in users details if they need that detail.

[DrewAPicture]

Fixed docs

[DrewAPicture]

33008.diff​ fixes the docs formatting and adjusts the summary to more closely explain the point of firing.

[SergeyBiryukov]

In 33732:

Multisite: Add 'invite_user' action that fires immediately after a user is invited to join a site, but before the notification is sent.

props ebinnion, MikeHansenMe, DrewAPicture.
fixes #33008.