Add missing comment to RateLimiter::Request() (#9392)

Summary:
**Context/Summary:**
There are two `RateLimiter::Request()` in public header. One of them is missing some comment that the other one has.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9392

Test Plan: rely on CI test

Reviewed By: pdillinger

Differential Revision: D33623609

Pulled By: hx235

fbshipit-source-id: 42dc06308ff0bcf5ee7ef67e0b1c0172fc239b20

include/rocksdb/rate_limiter.h

@@ -59,13 +59,15 @@ class RateLimiter : public Customizable {
   // Request for token for bytes. If this request can not be satisfied, the call
   // is blocked. Caller is responsible to make sure
   // bytes <= GetSingleBurstBytes()
+  // and bytes >= 0.
   virtual void Request(const int64_t /*bytes*/, const Env::IOPriority /*pri*/) {
     assert(false);
   }
 
   // Request for token for bytes and potentially update statistics. If this
   // request can not be satisfied, the call is blocked. Caller is responsible to
-  // make sure bytes <= GetSingleBurstBytes().
+  // make sure bytes <= GetSingleBurstBytes()
+  // and bytes >= 0.
   virtual void Request(const int64_t bytes, const Env::IOPriority pri,
                        Statistics* /* stats */) {
     // For API compatibility, default implementation calls the older API in