I don't know if it is possible to add haddock to functions whose type signatures are generated by template haskell.

Could the documentation be an argument of mkLenses?

Does haddock run on the template-haskell expanded code?

TH macros must have type Q [Dec]. Dec has no constructor for comments, with the exception of pragmas. This might be feature request worthy, though it is a rather strange case to want to generate comments via a macro.