`absl::ScopedMockLog`: Explicitly document that it captures logs emitted by all threads

This is already documented by a [test case](https://github.com/abseil/abseil-cpp/blob/fd8b35b9aa409759d9c5b070a5604849c0a274cc/absl/log/scoped_mock_log_test.cc#L218), but it would have been nicer to just read about it in the header file.

PiperOrigin-RevId: 719021749
Change-Id: I058eeec77d6c54b906ad449221b09222be59f898

absl/log/scoped_mock_log.h

diff --git a/absl/log/scoped_mock_log.h b/absl/log/scoped_mock_log.h
index 399e604..a383066 100644
--- a/absl/log/scoped_mock_log.h
+++ b/absl/log/scoped_mock_log.h
@@ -40,8 +40,8 @@
 
 // ScopedMockLog
 //
-// ScopedMockLog is a LogSink that intercepts LOG() messages issued during its
-// lifespan.
+// ScopedMockLog is a LogSink that intercepts LOG() messages issued by all
+// threads when active.
 //
 // Using this together with GoogleTest, it's easy to test how a piece of code
 // calls LOG(). The typical usage, noting the distinction between