Add comment about how fs_scratch is protecting us

include/rocksdb/file_system.h

@@ -867,11 +867,16 @@ struct FSReadRequest {
   // 2. Take ownership of the object managed by fs_scratch.
   // 3. Handle invoking the custom deleter function from the FSAllocationPtr.
   //
-  // WARNING: Do NOT assume that fs_scratch points to the start of the actual
+  // WARNING 1: Do NOT assume that fs_scratch points to the start of the actual
   // char* data returned by the read. As the type signature suggests, fs_scratch
   // is a pointer to any arbitrary data type. Use result.data() to get a valid
   // start to the real data. See https://github.com/facebook/rocksdb/pull/13189
   // for more context.
+  //
+  // WARNING 2: Since fs_scratch is a unique pointer, FSReadRequest's copy
+  // constructor is implicitly disabled. This turns out to be very useful
+  // because we want users to be explicit when setting offset, len, and
+  // optional_read_size
   FSAllocationPtr fs_scratch;
 };