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/isa/aarch64/abi.rs

@@ -86,9 +86,9 @@
 //!   formal arguments, would:
 //!   - Accept a pointer P to the struct return area in x0 on entry.
 //!   - Return v3 in x0.
-//!   - Return v2 in memory at [P].
-//!   - Return v1 in memory at [P+8].
-//!   - Return v0 in memory at [P+16].
+//!   - Return v2 in memory at `[P]`.
+//!   - Return v1 in memory at `[P+8]`.
+//!   - Return v0 in memory at `[P+16]`.
 
 use crate::ir;
 use crate::ir::types;