Add collaborative tags documentation

[juliusknorr]

The documentation misses to explain the basic idea of collaborative tags. While they are used with access control, automated tagging and retention, there should be some basic notes on what the collaborative tags are for and what the restricted and invisible settings do.

[joshtrichards]

We should also clean-up the terms.

Sometimes we say different things and it's not clear if they're the same or different - i.e.

• "tags"
• "system tags"
• "collaborative tags"

Also will make our lives easier when/if things like individual account (private?) and/or group-level tags become a thing.