[doc] explain how case clauses with multiple selectors are supported

src/lib.rs

@@ -1466,6 +1466,22 @@ pub enum Statement {
  reject: Block,
  },
  /// Conditionally executes one of multiple blocks, based on the value of the selector.
+ ///
+ /// Each case must have a distinct [`value`], exactly one of which must be
+ /// [`Default`]. The `Default` may appear at any position, and covers all
+ /// values not explicitly appearing in other cases. A `Default` appearing in
+ /// the midst of the list of cases does not shadow the cases that follow.
+ ///
+ /// Some backend languages don't support fallthrough (HLSL due to FXC,
+ /// WGSL), and may translate fallthrough cases in the IR by duplicating
+ /// code. However, all backend languages do support cases selected by
+ /// multiple values, like `case 1: case 2: case 3: { ... }`. This is
+ /// represented in the IR as a series of fallthrough cases with empty
+ /// bodies, except for the last.
+ ///
+ /// [`value`]: SwitchCase::value
+ /// [`body`]: SwitchCase::body
+ /// [`Default`]: SwitchValue::Default
  Switch {
  selector: Handle<Expression>, //int
  cases: Vec<SwitchCase>,