Made several touchups to semantics.md.
Made several touchups to semantics.md.

In particular, removed the warning that says it doesn't reflect reality.

Fixes #18.

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