Document the use of mandatory macro parameters and add named macro tokens

[bernt-matthias]

learned while working on https://github.com/galaxyproject/galaxy/pull/13152

[bernt-matthias]

I hope that I have addressed all comment.

Just had the idea that one could allow content to yield tags and interpret it as default if the corresponding token is not given. Seems that writing docs is not only annoying but may result in better ideas and helps to reason about code.

[bernt-matthias]

Hey @nsoranzo there were quite a few commits after your approval. Are you still OK with merging?

[jmchilton]

Sorry for the delay on the merge - very nice work. Thank you both!