Add help to hir_analysis_unrecognized_intrinsic_function

To help remind forgetful people like me what step they forgot.

compiler/rustc_hir_analysis/messages.ftl

@@ -462,6 +462,7 @@ hir_analysis_unrecognized_atomic_operation =
 hir_analysis_unrecognized_intrinsic_function =
     unrecognized intrinsic function: `{$name}`
     .label = unrecognized intrinsic
+    .help = if you're adding an intrinsic, be sure to update `check_intrinsic_type`
 
 hir_analysis_unused_associated_type_bounds =
     unnecessary associated type bound for not object safe associated type

compiler/rustc_hir_analysis/src/errors.rs

@@ -143,6 +143,7 @@ pub struct WrongNumberOfGenericArgumentsToIntrinsic<'a> {
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_unrecognized_intrinsic_function, code = E0093)]
+#[help]
 pub struct UnrecognizedIntrinsicFunction {
     #[primary_span]
     #[label]