Clarification regarding record format

Summary:
The CRC is actually calculated based on the record type and payload.
The wiki should also be updated accordingly and extended with a section on the recyclable record format.
Closes https://github.com/facebook/rocksdb/pull/3576

Differential Revision: D7196478

Pulled By: siying

fbshipit-source-id: 39f7a0395075cc73e2aa2bfc9e42c85bce35e765

db/log_writer.h

@@ -49,7 +49,7 @@ namespace log {
  * |CRC (4B) | Size (2B) | Type (1B) | Payload   |
  * +---------+-----------+-----------+--- ... ---+
  *
- * CRC = 32bit hash computed over the payload using CRC
+ * CRC = 32bit hash computed over the record type and payload using CRC
  * Size = Length of the payload data
  * Type = Type of record
  *        (kZeroType, kFullType, kFirstType, kLastType, kMiddleType )