document the use-cases of DenseBitSet a bit more

Author: lqd

compiler/rustc_index/src/bit_set.rs

@@ -97,7 +97,13 @@ macro_rules! bit_relations_inherent_impls {
 
 /// A fixed-size bitset type with a dense representation.
 ///
-/// NOTE: Use [`GrowableBitSet`] if you need support for resizing after creation.
+/// Note 1: Since this bitset is dense, if your domain is big, and/or relatively
+/// homogeneous (for example, with long runs of bits set or unset), then it may
+/// be preferable to instead use a [MixedBitSet], or an
+/// [IntervalSet](crate::interval::IntervalSet). They should be more suited to
+/// sparse, or highly-compressible, domains.
+///
+/// Note 2: Use [`GrowableBitSet`] if you need support for resizing after creation.
 ///
 /// `T` is an index type, typically a newtyped `usize` wrapper, but it can also
 /// just be `usize`.