Add reentrancy comment for critical section.

Change-Id: I921484802d8c73c5aa28138726a8826cc3689fd8

Bug: webrtc:10347
Change-Id: I921484802d8c73c5aa28138726a8826cc3689fd8
Reviewed-on: https://webrtc-review.googlesource.com/c/123767
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Ruslan Burakov <kuddai@google.com>
Cr-Commit-Position: refs/heads/master@{#26856}

rtc_base/critical_section.h

@@ -52,7 +52,7 @@ namespace rtc {
 
 // Locking methods (Enter, TryEnter, Leave)are const to permit protecting
 // members inside a const context without requiring mutable CriticalSections
-// everywhere.
+// everywhere. CriticalSection is reentrant lock.
 class RTC_LOCKABLE CriticalSection {
  public:
   CriticalSection();