(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.
"cases" is ambiguous, since a coupel of paragraphs earlier there are other "cases" when conversion takes place (say in a type when required and possible). Maybe use a term like "language feature", or "operator" instead.