doc: build: dts: bindings-syntax: clarify compat matching

Clarify the identification and matching of bindings to devicetree
node compats. The previous definition implicitly hinted at two
bindings with the same compat being allowed, but did not state this
clearly. The updated wording should make it clear exactly how
bindings may be defined, and how they are matched.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>