doc: dts: bindings: Document regex support and clean up a bit

 - Regexes (e.g. 'pinctrl-.*') are supported in property names in
   bindings, but it was undocumented

 - Remove the mention of 'generation: structure'. The only implemented
   output format is #define's.

 - Clean up the language a bit and move related stuff so that it appears
   together

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>