[MOD]Extend info for new admins about why categories are useful

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47628 b456876b-0849-0410-b77d-98878d47e9d5
[FIX]Short description of structures

git-svn-id: file:///var/local/code.tiki.org/tikisvn/trunk@47629 b456876b-0849-0410-b77d-98878d47e9d5