devicetree: document limitations related to for-each macros

Document a C preprocessor limitation that prevents "nested" use of
for-each style devicetree macros in the way users sometimes expect.

Put this in the devicetree.h header where it will show up in the
detailed doxygen description in our HTML documentation, in addition to
being in the sources themselves for people who prefer to look for
docstrings there.

Signed-off-by: Martí Bolívar <marti.bolivar@oss.qualcomm.com>