add comment to RawVec::cap field

Author: the8472

library/alloc/src/raw_vec.rs

@@ -60,6 +60,11 @@ impl Cap {
 #[allow(missing_debug_implementations)]
 pub(crate) struct RawVec<T, A: Allocator = Global> {
     ptr: Unique<T>,
+    /// Never used for ZSTs; it's `capacity()`'s responsibility to return usize::MAX in that case.
+    ///
+    /// # Safety
+    ///
+    /// `cap` must be in the `0..=isize::MAX` range.
     cap: Cap,
     alloc: A,
 }