Clarify more MIR docs

Author: JakobDegen

compiler/rustc_middle/src/mir/mod.rs

@@ -1561,6 +1561,8 @@ impl Statement<'_> {
 #[derive(Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable, TypeFoldable)]
 pub enum StatementKind<'tcx> {
     /// Write the RHS Rvalue to the LHS Place.
+    ///
+    /// The LHS place may not overlap with any memory accessed on the RHS.
     Assign(Box<(Place<'tcx>, Rvalue<'tcx>)>),
 
     /// This represents all the reading that a pattern match may do
@@ -1778,6 +1780,19 @@ static_assert_size!(Place<'_>, 16);
 pub enum ProjectionElem<V, T> {
     Deref,
     Field(Field, T),
+    /// Index into a slice/array.
+    ///
+    /// Note that this does not also dereference, and so it does not exactly correspond to slice
+    /// indexing in Rust. In other words, in the below Rust code:
+    ///
+    /// ```rust
+    /// let x = &[1, 2, 3, 4];
+    /// let i = 2;
+    /// x[i];
+    /// ```
+    ///
+    /// The `x[i]` is turned into a `Deref` followed by an `Index`, not just an `Index`. The same
+    /// thing is true of the `ConstantIndex` and `Subslice` projections below.
     Index(V),
 
     /// These indices are generated by slice patterns. Easiest to explain