Added a warning about the behavior of DirInode.ReadEntries.
Added a warning about the behavior of DirInode.ReadEntries.

jacobsa/ratelimit,GoogleCloudPlatform/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,mbrukman/gcsfuse,GoogleCloudPlatform/gcsfuse,BanzaiMan/gcsfuse,jacobsa/timeutil,kahing/gcsfuse