Auto merge of #3430 - RalfJung:doc, r=RalfJung

make some doc comments not doc tests

`./miri test --doc` will run doctests even if we have them disabled (that's a cargo quirk: https://github.com/rust-lang/cargo/issues/13668). This fixes that command to not fail.

src/tools/miri/src/borrow_tracker/tree_borrows/diagnostics.rs

@@ -365,7 +365,7 @@ type S = &'static str;
 /// Pretty-printing details
 ///
 /// Example:
-/// ```
+/// ```rust,ignore (private type)
 /// DisplayFmtWrapper {
 ///     top: '>',
 ///     bot: '<',
@@ -393,7 +393,7 @@ struct DisplayFmtWrapper {
 /// Formating of the permissions on each range.
 ///
 /// Example:
-/// ```
+/// ```rust,ignore (private type)
 /// DisplayFmtPermission {
 ///     open: "[",
 ///     sep: "|",
@@ -425,7 +425,7 @@ struct DisplayFmtPermission {
 /// Formating of the tree structure.
 ///
 /// Example:
-/// ```
+/// ```rust,ignore (private type)
 /// DisplayFmtPadding {
 ///     join_middle: "|-",
 ///     join_last: "'-",
@@ -463,7 +463,7 @@ struct DisplayFmtPadding {
 /// How to show whether a location has been accessed
 ///
 /// Example:
-/// ```
+/// ```rust,ignore (private type)
 /// DisplayFmtAccess {
 ///     yes: " ",
 ///     no: "?",

src/tools/miri/src/borrow_tracker/tree_borrows/unimap.rs

@@ -132,7 +132,7 @@ where
     /// the associated `UniIndex` from ALL `UniValMap`s.
     ///
     /// Example of such behavior:
-    /// ```
+    /// ```rust,ignore (private type can't be doctested)
     /// let mut keymap = UniKeyMap::<char>::default();
     /// let mut valmap = UniValMap::<char>::default();
     /// // Insert 'a' -> _ -> 'A'