Updating dox for mod_rest. Ideas expressed / clarified: 1) Making clear that mod_rest isn't to be installed under VirtualHosts AND as a component. 2) Understanding some of the implications of this choice: A) Changes to user authentication B) How it affects subdomains 3) More consistent use of domain names for clarity. 4) Using different heading sizes to show scope of section. Essentially, I added all the tidbits I had to clarify in getting this to work in my own example.

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summary: Lets users create and join roster groups
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Introduction
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This module lets you join groups using an ad-hoc command. When a user
joins a group, everyone in the group is added to their roster, and they
are added to the rosters of existing members.

TODO
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-   Leaving groups
-   Add a roster group/tag when adding roster items (tricky with current
    rostermanager API)