Add rule identifiers for unassociated must_use notes

We have two note admonitions at the bottom of the `must_use` section
that weren't associated with any particular rule but, by falling
below the final rule without identifiers of their own, appeared to
be associated with it.  Let's give each of these a rule identifier to
make this more clear.

Author: traviscross

src/attributes/diagnostics.md

@@ -495,6 +495,7 @@ impl Tr for () {
 > [!NOTE]
 > `rustc` lints against use on functions in trait implementations. This may become an error in the future.
 
+r[attributes.diagnostics.must_use.wrapping-suppression]
 > [!NOTE]
 > Wrapping the result of a `#[must_use]` function in certain expressions can suppress the [fn-based check][attributes.diagnostics.must_use.fn], because the [expression] of the [expression statement] is not a [call expression] or [method call expression] to a `#[must_use]` function.  The [type-based check][attributes.diagnostics.must_use.type] still applies if the type of the overall expression is `#[must_use]`.
 >
@@ -526,6 +527,7 @@ impl Tr for () {
 > if true { g() } else { MustUse }; // ERROR: Must be used.
 > ```
 
+r[attributes.diagnostics.must_use.underscore-idiom]
 > [!NOTE]
 > Using a [let statement] or [destructuring assignment] with a pattern of `_` when a must-used value is purposely discarded is idiomatic.
 >