macros.ddoc

As mentioned here: https://github.com/D-Programming-Language/phobos/pull/2682#issuecomment-62559462

I couldn't get `apidocs-*` to build, so that's untested. Judging from the current website, they're missing `macros.ddoc`, too: http://dlang.org/library/std/signals.html has no commas in "properties signals and slots etc".