doc: dts: macros.bnf: add docs for new for-each macros

Document the new DT_FOREACH_OKAY_INST_<compat> and
DT_FOREACH_OKAY_INST_VARGS_<compat> macros that are generated by
gen_defines.py and explain how they are used in devicetree.h.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>