One of our users today asked us if dashes were allowed in recording rule names.
We asserted that they were not, but also that we could not remember for certain.
After determining empirically that they are _not_ allowed, I realized that the
documentation could be slightly clearer about valid rule names.
This PR simply adds a note to the documentation re-iterating that the rules must
be valid metric names - and more importantly, adds a link to where a user can
read what those *are*, in case they were not aware (or did not know where to find it).
Signed-off-by: Andrew Hayworth <ahayworth@gmail.com>
