`cleanup`: specify that it's safe to use the class in a signal handler.

The class is intentionally designed for exactly what is needed here: no type
erasure, and therefore allocations and no locks taken. It's also not conceivable
that we would ever change this without introducing a new API, as doing so would
serve no purpose and harm performance.

PiperOrigin-RevId: 852955861
Change-Id: I67bb75cf17c1184392bbec6ed9d15faee2f6376b

absl/cleanup/cleanup.h

diff --git a/absl/cleanup/cleanup.h b/absl/cleanup/cleanup.h
index 311e482..632ec6e 100644
--- a/absl/cleanup/cleanup.h
+++ b/absl/cleanup/cleanup.h
@@ -19,6 +19,10 @@
 // `absl::Cleanup` implements the scope guard idiom, invoking the contained
 // callback's `operator()() &&` on scope exit.
 //
+// This class doesn't allocate or take any locks, and is safe to use in a signal
+// handler. Of course the callback with which it is constructed also must be
+// signal safe in order for this to be useful.
+//
 // Example:
 //
 // ```