Add test for disambiguator mismatch with crate

This currently calls `std` a "crate" in one part of the message and a
"module" in another part. The next commits fix this so it says "crate"
in both places.

Author: camelid

src/test/rustdoc-ui/intra-doc/disambiguator-mismatch.rs

@@ -73,4 +73,9 @@ trait T {}
 //~^ ERROR incompatible link kind for `f`
 //~| NOTE this link resolved
 //~| HELP add parentheses
+
+/// Link to [fn@std]
+//~^ ERROR unresolved link to `std`
+//~| NOTE this link resolves to the crate `std`
+//~| HELP to link to the module, prefix with `mod@`
 pub fn f() {}

src/test/rustdoc-ui/intra-doc/disambiguator-mismatch.stderr

@@ -138,5 +138,16 @@ LL - /// Link to [const@f]
 LL + /// Link to [f()]
    | 
 
-error: aborting due to 12 previous errors
+error: unresolved link to `std`
+  --> $DIR/disambiguator-mismatch.rs:77:14
+   |
+LL | /// Link to [fn@std]
+   |              ^^^^^^ this link resolves to the crate `std`, which is not in the value namespace
+   |
+help: to link to the module, prefix with `mod@`
+   |
+LL | /// Link to [mod@std]
+   |              ~~~~
+
+error: aborting due to 13 previous errors