scripts: gen_defines: add some missing comments

We generally try to have comments in this file that show the form of
each generated macro. This is particularly important in the
write_vanilla_props() function, since that is called on every node in
the tree and handles generic macros that are widely applicable.

Various generated macros have been added over time that don't have
the corresponding comments; add these now.

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