Auto merge of #2706 - alx741:extend_manpages, r=alexcrichton

Auto merge of #2706 - alx741:extend_manpages, r=alexcrichton

Add man pages for cargo commands

So, the markdown port (#2672) is in idle, I'll be working in a way to make it without depending on external tools, but that is going to take a bit longer.

These are the **troff** pages for the rest of the cargo commands; previously the *cargo-build(1)* was added, so here are the rest.

No extra build dependencies here, just the new man pages.