module file support: major rework of docs (#2169)

* module file support: major rework of docs

* module file support: fixed issues found by @adamjstewart

- list or enumeration should not be indented
- use console instead of bash or csh in things that are not scripts
- other typos

* module file support: fixed other issues found by @adamjstewart

- tables should not be indented
- substitute lines with pyobject to import an entire function
- get help output running commands
- typos

* module file support: fixes according to review comments

- @citibeth moved `spack module loads` after `spack load`
- @glennpj tried to clarify installation table + changes to language
- @tgamblin Removed top level section and moved the whole thing into the reference manual

* module file support: moved directive before spack module loads