spack uninstall: improve help message

[alalazo]

fixes #12527

• Mention that specs can be uninstalled by hash also in the help message
• Reference spack gc in case people are looking for ways to clean the store from build time dependencies.
• Use "spec" instead of "package" to avoid ambiguity in the error message.

The result is the following:

$ spack uninstall -h
usage: spack uninstall [-hfRya] ...

remove installed packages

positional arguments:
  installed_specs   one or more installed package specs

optional arguments:
  -h, --help        show this help message and exit
  -f, --force       remove regardless of whether other packages or environments depend on this one
  -R, --dependents  also uninstall any packages that depend on the ones given via command line
  -y, --yes-to-all  assume "yes" is the answer to every confirmation request
  -a, --all         remove ALL installed packages that match each supplied spec

Specs to be uninstalled are specified using the spec syntax (`spack help --spec`) and can be identified by their hashes. To remove packages that are needed only at build time and were not explicitly installed see `spack gc -h`.

When using the --all option ALL packages matching the supplied specs will be uninstalled. For instance, `spack uninstall --all libelf` uninstalls all the versions of `libelf` currently present in Spack's store. If no spec is supplied, all installed packages will be uninstalled. If used in an environment, all packages in the environment will be uninstalled.

[alalazo]

@ejcjason @RobertRosca Following #16863 here's the other part of the request in #12527. Let me know if you think anything else is needed to help the user experience.

[alalazo]

Thanks @adamjstewart