Expand comment on NonMutatingUseContext.

Author: cjgillot

compiler/rustc_middle/src/mir/visit.rs

@@ -1252,6 +1252,9 @@ pub enum NonMutatingUseContext {
     /// AddressOf for *const pointer.
     AddressOf,
     /// PlaceMention statement.
+    ///
+    /// This statement is executed as a check that the `Place` is live without reading from it,
+    /// so it must be considered as a non-mutating use.
     PlaceMention,
     /// Used as base for another place, e.g., `x` in `x.y`. Will not mutate the place.
     /// For example, the projection `x.y` is not marked as a mutation in these cases: