Env help: expand and refine subcommand help and descriptions (#47089)

This PR is in response to a question in the `environments` slack channel (https://spackpm.slack.com/archives/CMHK7MF51/p1729200068557219) about inadequate CLI help/documentation for one specific subcommand.

This PR uses the approach I took for the descriptions and help for `spack test` subcommands.  Namely, I use the first line of the relevant docstring as the description, which is shown per subcommand in `spack env -h`, and the entire docstring as the help.  I then added, where it seemed appropriate, help.  I also tweaked argument docstrings to tighten them up, make consistent with similar arguments elsewhere in the command, and elaborate when it seemed important.  (The only subcommand I didn't touch is `loads`.)

For example, before:
```
$ spack env update -h
usage: spack env update [-hy] env

positional arguments:
  env               name or directory of the environment to activate

optional arguments:
  -h, --help        show this help message and exit
  -y, --yes-to-all  assume "yes" is the answer to every confirmation request
```

After the changes in this PR:
```
$ spack env update -h
usage: spack env update [-hy] env

update the environment manifest to the latest schema format

    update the environment to the latest schema format, which may not be
    readable by older versions of spack

    a backup copy of the manifest is retained in case there is a need to revert
    this operation
    

positional arguments:
  env               name or directory of the environment

optional arguments:
  -h, --help        show this help message and exit
  -y, --yes-to-all  assume "yes" is the answer to every confirmation request
```

---------

Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>

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