The Enhance dialog should explain why Denoise or Raw Details are greyed out by providing a simple explanation:
Denoise and Raw Details don't work on the selected file types: <file types>
where <file types> provides the precise file formats of the selected files: mRAW, sRAW, linear DNG, JPEG, TIFF, PSD, PSB, etc. LR has that information internally -- it just needs to show it to the user.
In addition, Learn More should link to a page that provides a comprehensible explanation. Right now, it just says "any Bayer or X-Trans RAW files" -- how many non-nerd users know what those are?
This trivial change would avoid hours of user frustrations and needless FAQs here.