Standardize argparse help messages (#2847)

bin/spack

lib/spack/spack/cmd/activate.py

lib/spack/spack/cmd/arch.py

lib/spack/spack/cmd/bootstrap.py

lib/spack/spack/cmd/build.py

lib/spack/spack/cmd/cd.py

lib/spack/spack/cmd/checksum.py

lib/spack/spack/cmd/clean.py

lib/spack/spack/cmd/common/arguments.py

lib/spack/spack/cmd/compiler.py

lib/spack/spack/cmd/compilers.py

lib/spack/spack/cmd/config.py

lib/spack/spack/cmd/configure.py

lib/spack/spack/cmd/create.py

lib/spack/spack/cmd/deactivate.py

lib/spack/spack/cmd/debug.py

lib/spack/spack/cmd/dependents.py

lib/spack/spack/cmd/diy.py

lib/spack/spack/cmd/doc.py

lib/spack/spack/cmd/edit.py