update available_parallelism docs since cgroups and sched_getaffinity are now taken into account

Author: the8472

library/std/src/thread/mod.rs

@@ -1524,7 +1524,10 @@ fn _assert_sync_and_send() {
 ///
 /// On Linux:
 /// - It may overcount the amount of parallelism available when limited by a
-///   process-wide affinity mask, or when affected by cgroup limits.
+///   process-wide affinity mask or cgroup quotas and cgroup2 fs or `sched_getaffinity()` can't be
+///   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.
 ///
 /// On all targets:
 /// - It may overcount the amount of parallelism available when running in a VM