adding more description to binary caches (#23934)
adding more description to binary caches (#23934)

It is currently kind of confusing to the reader to distinguish spack buildcache install
and spack install, and it is not clear how to use a build cache once a mirror is added.
Hopefully this little big of description can help (and I hope I got it right!)

Signed-off-by: vsoch <7cb4235d215b6aa84109596e3788aeb2b4ed1c44@users.noreply.github.com>

Co-authored-by: vsoch <7cb4235d215b6aa84109596e3788aeb2b4ed1c44@users.noreply.github.com>
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack