Minor fixups related to ambiguity errors

diff --git a/compiler/rustc_expand/src/mbe/diagnostics.rs b/compiler/rustc_expand/src/mbe/diagnostics.rs
index 5a88d57..c53c3e0 100644
--- a/compiler/rustc_expand/src/mbe/diagnostics.rs
+++ b/compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -221,7 +221,7 @@ fn after_arm(&mut self, which_matcher: WhichMatcher, result: &NamedParseResult<S
             }
             Ambiguity => {
                 if self.result.is_none() {
-                    bug!("`Error(..)` is only constructed through `Self::ambiguity()`");
+                    bug!("An ambiguity error occurred but `Self::ambiguity()` was not called");
                 }
             }
             ErrorReported(guar) => self.result = Some((self.root_span, guar)),

diff --git a/compiler/rustc_expand/src/mbe/macro_parser.rs b/compiler/rustc_expand/src/mbe/macro_parser.rs
index 641c50b..22f27df 100644
--- a/compiler/rustc_expand/src/mbe/macro_parser.rs
+++ b/compiler/rustc_expand/src/mbe/macro_parser.rs

@@ -302,6 +302,8 @@ pub(crate) enum ParseResult<T, F> {
     /// The usize is the approximate position of the token in the input token stream.
     Failure(F),
     /// The input could be parsed in multiple distinct ways.
+    ///
+    /// [`Tracker::ambiguity()`] will be called beforehand.
     Ambiguity,
     ErrorReported(ErrorGuaranteed),
 }

diff --git a/compiler/rustc_expand/src/mbe/macro_rules.rs b/compiler/rustc_expand/src/mbe/macro_rules.rs
index a6acccd..43863b3 100644
--- a/compiler/rustc_expand/src/mbe/macro_rules.rs
+++ b/compiler/rustc_expand/src/mbe/macro_rules.rs

@@ -374,6 +374,9 @@ pub(super) trait Tracker<'matcher> {
     /// this is called, `before_match_loc` was called at least once (with a `MatcherLoc::Eof`).
     fn after_arm(&mut self, which_matcher: WhichMatcher, result: &NamedParseResult<Self::Failure>);
 
+    /// An ambiguity error occurred.
+    ///
+    /// The parser will return [`NamedParseResult::Ambiguity`] after calling this.
     fn ambiguity(
         &mut self,
         parser: &Parser<'_>,