Expand doc on dealing with "proc macro not expanded"

Added the error message to the doc for the UnresolvedProcMacro
diagnostic, explaining that either enabling the procMacro setting
or disabling this diagnostic should make the warnings go away.

crates/hir_def/src/diagnostics.rs

@@ -133,6 +133,10 @@ impl Diagnostic for InactiveCode {
 // This diagnostic is shown when a procedural macro can not be found. This usually means that
 // procedural macro support is simply disabled (and hence is only a weak hint instead of an error),
 // but can also indicate project setup problems.
+//
+// If you are seeing a lot of "proc macro not expanded" warnings, you can add this option to the
+// `rust-analyzer.diagnostics.disabled` list to prevent them from showing. Alternatively you can
+// enable support for procedural macros (see `rust-analyzer.procMacro.enable`).
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct UnresolvedProcMacro {
     pub file: HirFileId,