Add note to the docs on is_from_proc_macro

Author: Jarcho

clippy_utils/src/check_proc_macro.rs

@@ -271,7 +271,10 @@ impl_with_search_pat!(LateContext: ImplItem with impl_item_search_pat);
 impl_with_search_pat!(LateContext: FieldDef with field_def_search_pat);
 impl_with_search_pat!(LateContext: Variant with variant_search_pat);
 
-/// Checks if the item likely came from a proc-macro
+/// Checks if the item likely came from a proc-macro.
+///
+/// This should be called after `in_external_macro` and the initial pattern matching of the ast as
+/// it is significantly slower than both of those.
 pub fn is_from_proc_macro<T: WithSearchPat>(cx: &T::Context, item: &T) -> bool {
     let (start_pat, end_pat) = item.search_pat(cx);
     !span_matches_pat(cx.sess(), item.span(), start_pat, end_pat)