The `hasValuelessStatics` function in `semtypinst.nim` only checked for
`tyStatic`, missing `tyTypeDesc(tyGenericParam)`. This caused
`sizeof(T)` inside a typedesc template called from a generic type's
`when` clause to error with "'sizeof' requires '.importc' types to be
'.completeStruct'".
The fix adds a check for `tyTypeDesc` wrapping `tyGenericParam`,
recognizing it as an unresolved generic parameter that needs resolution
before evaluation.
Also documents the `completeStruct` pragma in the manual.
