Add docs notice that API activity for group-level IP address restriction was added in 12.3

Overview

@marcel.amirault noted in gitlab-com/www-gitlab-com!31421 (comment 233054961) that the docs currently reads:

Restriction currently applies to:

• UI.
• API access.
• From GitLab 12.4, Git actions via SSH.

API access was added in 12.3 (!15282 (merged)).

Proposal

Change the above and link out to #12874 (closed):

Restriction currently applies to:

• UI.
• From GitLab 12.3, API access.
• From GitLab 12.4, Git actions via SSH.