(PUP-2663) Specify String<->Numeric conversion

This cleans up the specification of conversion between string and
numeric values. It also updates it to specify the grammar of strings
that are able to be converted, which is what was implemented by
PUP-2663.
(PUP-2663) Reference conversion rules

The areas that specify when operators convert no longer referenced how
those conversions would be done. This updates it to reference the
conversion section.
(PUP-2663) Clarify ambiguous language

The language around when conversions could actually happen was slightly
ambiguous. This makes it very clear that it only happens in certain
cases and removes the wording about when it doesn't happen so that there
isn't any confusion when the list isn't complete.
Merge pull request #2 from zaphod42/issue/master/pup-2663-specify-numeric-conversion

(PUP-2663) Specify String<->Numeric conversion