Update G.pod

"The DSL form may or may not be defined if a symbol does not have
an explicit name." — implies "If a symbol does not have an explicit name, its DSL form is undefined?" 

[Looks like](https://gist.github.com/rns/8372608) DSL form is currently only defined for character classes and every unique character in single-quoted strings.

Perhaps some rewording? It feels a little bit vague, I'm afraid.