Improve documentation of constants throughout the codebase

Improve documentation of constants throughout the codebase

The most notable improvements I was able to fit into this patch can be
seen in the User class, as well as in AbstractRestriction.

Our documentation generator ignores the @const tag. It's not needed. Just
have a comment above a constant and it will show up in the generated
documentation.

Using @var is misleading because a constant is not a "variable". The type
of a constant is strictly derived from it's value. Documenting the type
typically does not provide useful information. Doxygen does not understand
the type, but ignores any @… tag and renders everything else as plain text.

I can split this patch if you prefer. Please tell me.

Change-Id: I8019ae45c049822cdc1768d895ea3e3216c6db5f