Add a comment to `Arc::MAX_REFCOUNT`

The constant name `MAX_REFCOUNT` suggests that the value is a
_hard_ limit on the amount of references to an `Arc`. This is
a more soft limit however. This commit adds a comment to the
constant to annotate this.

See also: PR #37605

src/liballoc/arc.rs

@@ -36,6 +36,10 @@
 use core::convert::From;
 use heap::deallocate;
 
+/// A soft limit on the amount of references that may be made to an `Arc`.
+///
+/// Going above this limit will abort your program (although not
+/// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 
 /// A thread-safe reference-counting pointer.