branches: 1.15.24;
Move common/contents file into MD dirs again.
I know it isn't a good thing to have many copy-and-pasted contents
(set file descriptions etc) in all MD files, but it's much more annoying
to maintain a bunch of .if \n [${MACHINE}] conditionals for all ports
and maintainers rarely notice that they also have to prepare or update
MD contents in MI content file with messy ifdefs.

XXX: of course common text should be shared by mdoc macro etc.
but I doubt playing mdoc(7) is really worth...