Member docs: Update docs on tags and mentions.
Rewrote it and corrected some outdated information about how to tag forums etc. Also cleaned up the markup a bit.
Added information and links about which apps and admin addons are required for given functionality.
Also added some images to help illustrate some subjects.
Uncertain about whether the information on how the autocomplete popup is populated is correct anymore. Underlined it and added a question mark to make sure it annoys someone (possibly me) to figure it out.