Add some line comments to enum CoverageKind

The actual motivation here is to prevent `rustfmt` from suddenly reformatting
these enum variants onto a single line, when they become slightly shorter in
the future.

But there's no harm in adding some helpful documentation at the same time.

compiler/rustc_middle/src/mir/coverage.rs

@@ -99,9 +99,13 @@ impl From<InjectedExpressionId> for ExpressionOperandId {
 pub enum CoverageKind {
     Counter {
         function_source_hash: u64,
+        /// ID of this counter within its enclosing function.
+        /// Expressions in the same function can refer to it as an operand.
         id: CounterValueReference,
     },
     Expression {
+        /// ID of this coverage-counter expression within its enclosing function.
+        /// Other expressions in the same function can refer to it as an operand.
         id: InjectedExpressionId,
         lhs: ExpressionOperandId,
         op: Op,