Improve S/// / s/// docs
Improve S/// / s/// docs

- Update to include changes brought by today's Rakudo fix:
    https://github.com/rakudo/rakudo/commit/97359ae42e
- Split s/// and S/// into separate sections so it's easier to
    understand the difference between usages, behaviour, and
    different return values.
- Fix incorrect info on s{} = '' form
- Use regex in examples instead of simple strings to make it
    more obvious the operators use regexes
- Include examples that use captures
- Reword sample string used to make it more obvious it's the actual
    sample string and not just some sort of abstract labels for
    parts of the operators
jonathanstowe/doc,dmaestro/doc,gfldex/doc,gfldex/doc,MasterDuke17/doc,gfldex/doc,tbrowder/doc,antquinonez/doc,gfldex/doc,stmuk/doc,jonathanstowe/doc,perl6/doc,jonathanstowe/doc,antquinonez/doc,perl6/doc,perl6/doc,tbrowder/doc,dmaestro/doc,antquinonez/doc,perl6/doc,jonathanstowe/doc,stmuk/doc,antquinonez/doc,gfldex/doc,tbrowder/doc,MasterDuke17/doc,stmuk/doc,dmaestro/doc,jonathanstowe/doc,antquinonez/doc,MasterDuke17/doc,dmaestro/doc,dmaestro/doc,MasterDuke17/doc,stmuk/doc,tbrowder/doc,stmuk/doc,perl6/doc