Fix doc warnings and link failures (#1948)

Also add configuration to CI to fail doc generation if any links are
broken. Unfortunately we can't blanket deny all warnings in rustdoc
since some are unconditional warnings, but for now this is hopefully
good enough.

Closes #1947

cranelift/codegen/src/ir/extfunc.rs

@@ -383,7 +383,7 @@ pub struct ExtFuncData {
     /// flag is best used when the target is known to be in the same unit of code generation, such
     /// as a Wasm module.
     ///
-    /// See the documentation for [`RelocDistance`](machinst::RelocDistance) for more details. A
+    /// See the documentation for [`RelocDistance`](crate::machinst::RelocDistance) for more details. A
     /// `colocated` flag value of `true` implies `RelocDistance::Near`.
     pub colocated: bool,
 }