Exclude enum's repr field from missing_docs

This is a synthetic field that isn't visible in the FFI module's source
code.

macro/src/expand.rs

@@ -296,6 +296,7 @@ fn expand_enum(enm: &Enum) -> TokenStream {
         #derives
         #[repr(transparent)]
         pub struct #ident {
+            #[allow(missing_docs)]
             pub repr: #repr,
         }
 

tests/ui/deny_missing_docs.stderr

@@ -31,14 +31,6 @@ error: missing documentation for a struct
   |
   = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: missing documentation for a struct field
- --> $DIR/deny_missing_docs.rs:9:1
-  |
-9 | #[cxx::bridge]
-  | ^^^^^^^^^^^^^^
-  |
-  = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
-
 error: missing documentation for an associated constant
  --> $DIR/deny_missing_docs.rs:9:1
   |