SH, MN10300 and cross-install documentation

[Alexandre Oliva]

In response to earlier requests by Gerald and Joseph, I went over the
documentation of the ports I maintain and of the cross tool-chain
installation instructions.  I didn't have the energy to write new text
for cross-building, but I could at least check its correctness and
remove outdated information.  Here's the result of this effort.  Ok to
install in 3.0 branch and mainline?