[clang][NFC] Add documentation for CastExpr::path(). (#85623)

This didn't have any documentation, so I had to do some experimenting in
godbolt when I used this in
#84138, and my reviewer later
also had some
[questions](#84138 (comment))
about this, so I figured it would be worth adding documentation.

clang/include/clang/AST/Expr.h

@@ -3562,6 +3562,18 @@ class CastExpr : public Expr {
   path_const_iterator path_begin() const { return path_buffer(); }
   path_const_iterator path_end() const { return path_buffer() + path_size(); }
 
+  /// Path through the class hierarchy taken by casts between base and derived
+  /// classes (see implementation of `CastConsistency()` for a full list of
+  /// cast kinds that have a path).
+  ///
+  /// For each derived-to-base edge in the path, the path contains a
+  /// `CXXBaseSpecifier` for the base class of that edge; the entries are
+  /// ordered from derived class to base class.
+  ///
+  /// For example, given classes `Base`, `Intermediate : public Base` and
+  /// `Derived : public Intermediate`, the path for a cast from `Derived *` to
+  /// `Base *` contains two entries: One for `Intermediate`, and one for `Base`,
+  /// in that order.
   llvm::iterator_range<path_iterator> path() {
     return llvm::make_range(path_begin(), path_end());
   }