rustc_mir_build: Clarify that 'mirrored' does not mean 'flipped' or '…

…reversed'

My intuition for 'mirrored' is that it means 'flipped' or 'reversed'.
Clarify that that is not what is meant to 'mirror' the THIR from the
HIR.

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -26,6 +26,14 @@ use crate::thir::cx::Cx;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
 impl<'tcx> Cx<'tcx> {
+    /// Create a THIR expression for the given HIR expression. This expands all
+    /// adjustments and directly adds the type information from the
+    /// `typeck_results`. See the [dev-guide] for more details.
+    ///
+    /// (The term "mirror" in this case does not refer to "flipped" or
+    /// "reversed".)
+    ///
+    /// [dev-guide]: https://rustc-dev-guide.rust-lang.org/thir.html
     pub(crate) fn mirror_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) -> ExprId {
         // `mirror_expr` is recursing very deep. Make sure the stack doesn't overflow.
         ensure_sufficient_stack(|| self.mirror_expr_inner(expr))