Include detail from header tooltips in module reference

[elstoc]

Processing module headers in darktable now contain a brief summary of the module along with details of the purpose, input, output and working color spaces.

Consider including this in the dtdocs module reference sections. Some of it is already included but not yet visible.

Should we use the yaml metadata to include this detail or include it as separate content?

[paperdigits]

Can you provide some samples of what these tooltips look like?

[elstoc]