[PATCH] doc: Fix parallel build of roff files

[David Bremner]

Austin Clements <amdragon at MIT.EDU> writes:

> The roff build rule builds all of the roff files in a single command.
> Previously, this was expressed as a multi-target rule, but since this
> is equivalent to specifying a copy of the rule for each target, make
> -jN could start up to N parallel instances of this command.  Fix this
> by bottlenecking this rule through a single stamp file.

pushed, with comment typofixed.

d