boulder: Add documentation for macros and keys

[ermo]

As part of this issue, move what is currently comments into a dedicated description: key per macro, so documentation can be shown by boulder help macros or similar?

Proposed example action macro documentation:

actions:

    - configure:
        description: Perform ./configure with the default options
        example: |
            setup:
                %configure \
                  --some-option \
                  --some-other-option
        command: |
            test -x ./configure || ( echo "%%configure: The ./configure script could not be found" ; exit 1 )
            ./configure %(options_configure)
        dependencies:
            - autoconf
            - automake