Clairify ast::PatKind::Struct docs.

The bool is mainly used for when a `..` is present, but it is also set on recovery to avoid errors.
The doc comment not describes both of these cases.

See https://github.com/rust-lang/rust/blob/cee794ee98d49b45a55ba225680d98e0c4672736/compiler/rustc_parse/src/parser/pat.rs#L890-L897 for the only place this is constructed.

compiler/rustc_ast/src/ast.rs

@@ -779,8 +779,8 @@ pub enum PatKind {
     Ident(BindingAnnotation, Ident, Option<P<Pat>>),
 
     /// A struct or struct variant pattern (e.g., `Variant {x, y, ..}`).
-    /// The `bool` is `true` in the presence of a `..`.
-    Struct(Option<P<QSelf>>, Path, ThinVec<PatField>, /* recovered */ bool),
+    /// The `bool` is `true` in the presence of a `..` (or when recovered).
+    Struct(Option<P<QSelf>>, Path, ThinVec<PatField>, bool),
 
     /// A tuple struct/variant pattern (`Variant(x, y, .., z)`).
     TupleStruct(Option<P<QSelf>>, Path, ThinVec<P<Pat>>),

compiler/rustc_parse/src/parser/pat.rs

@@ -890,6 +890,7 @@ impl<'a> Parser<'a> {
             e.span_label(path.span, "while parsing the fields for this pattern");
             e.emit();
             self.recover_stmt();
+            // When recovering, pretend we had `Foo { .. }`, to avoid cascading errors.
             (ThinVec::new(), true)
         });
         self.bump();
@@ -964,6 +965,8 @@ impl<'a> Parser<'a> {
     }
 
     /// Parses the fields of a struct-like pattern.
+    ///
+    /// The `bool` is `true` in the presence of a `..` at the end.
     fn parse_pat_fields(&mut self) -> PResult<'a, (ThinVec<PatField>, bool)> {
         let mut fields = ThinVec::new();
         let mut etc = false;