[review] mention that runtime may scale with # of mountpoints

Co-authored-by: Josh Triplett <josh@joshtriplett.org>

library/std/src/thread/mod.rs

@@ -1575,6 +1575,11 @@ fn _assert_sync_and_send() {
 ///   queried, e.g. due to sandboxing.
 /// - It may undercount the amount of parallelism if the current thread's affinity mask
 ///   does not reflect the process' cpuset, e.g. due to pinned threads.
+/// - If the process is in a cgroup v1 cpu controller, this may need to
+///   scan mountpoints to find the corresponding cgroup v1 controller,
+///   which may take time on systems with large numbers of mountpoints.
+///   (This does not apply to cgroup v2, or to processes not in a
+///   cgroup.)
 ///
 /// On all targets:
 /// - It may overcount the amount of parallelism available when running in a VM