Make it possible to document builtin macro definitions

[jplatte]

WIP (actual docs not written, CLI output not yet updated)
Closes #376.

[ermo]

@jplatte Per our understanding, the first pass of the defined-in-code internal macro documentation strings are now up for you to attempt to display via boulder recipe macros.

[ermo]

@jplatte:

Built-in definitions should be shown as %(name), not %name. Is this easy to change for you?

For context, think of %(definition) as a "constant" and %action as a "sub-routine" with a side-effect. This is of course entirely by convention for readability's sake.