Add stdlib docs to the Linux binary tarball.

Add stdlib docs to the Linux binary tarball.

These are not installed anywhere, but are included under `./doc` for
those who want an offline copy with their nightlies. This increases the
size of the (compressed) tarball from 76 to 83 MB.

I don't know if it's sane (or possible) to do something similar for the windows .exe/OSX .pkg things.