[Add]: Messages component explanation

[imath]

What needs to be done?

This documentation page just like #210 or #212 needs to be located into a components directory of the user docs and only deal with this component feature (for instance « Public message » is an Activity feature unlike what this codex page is saying). It should talk about the star feature. Well it needs to be completely reworked.

Handbook to add the page in

buddypress/docs/user

Tasks to accomplish

• This task has been added to the "Todo" panel of the Documentation project.
• A PR has been submitted to the BuddyPress GH repository, it has been linked to this issue/task & the task was moved to the "In progress" panel of the Documentation project.
• The PR has been converted to a SVN patch and committed to the BuddyPress SVN repository & the task was moved to the "Done" panel of the Documentation project.