Edit documentation to reflect recent change in forking

What does this MR do and why?

Edit documentation to reflect recent change in forking

Add info about possibility to select branches option while forking

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

go to https://docs.gitlab.com/ee/user/project/repository/forking_workflow.html and view the change there

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #24894 (closed)