Began on a relaxed guarantees section in semantics.md.
Began on a relaxed guarantees section in semantics.md.

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