doc: dts: Add lots of documentation for generated macros

Add detailed documentation for macros that get generated by
gen_defines.py. Covers properties, interrupts, phandle-arrays, clocks,
buses, flash partitions, SPI, etc.

Should be relatively complete now, though there might be overlooked
details.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
Co-Authored-By: Kumar Gala <kumar.gala@linaro.org>
Co-Authored-By: Marti Bolivar <marti.bolivar@nordicsemi.no>