Document RDB preamble in AOF rewrites in redis.conf.

redis.conf

@@ -755,6 +755,20 @@ auto-aof-rewrite-min-size 64mb
 # will be found.
 aof-load-truncated yes
 
+# When rewriting the AOF file, Redis is able to use an RDB preamble in the
+# AOF file for faster rewrites and recoveries. When this option is turned
+# on the rewritten AOF file is composed of two different stanzas:
+#
+#   [RDB file][AOF tail]
+#
+# When loading Redis recognizes that the AOF file starts with the "REDIS"
+# string and loads the prefixed RDB file, and continues loading the AOF
+# tail.
+#
+# This is currently turned off by default in order to avoid the surprise
+# of a format change, but will at some point be used as the default.
+aof-use-rdb-preamble no
+
 ################################ LUA SCRIPTING  ###############################
 
 # Max execution time of a Lua script in milliseconds.