don't mention sync.Locker in package documentation
don't mention sync.Locker in package documentation

(*Flock).Lock and (*Flock).Unlock return errors, unlike the Lock and
Unlock methods specified by sync.Locker. Therefore, don't mention
sync.Locker in package-level documentation, to avoid confusion.

Fixes #33.

gofrs/flock,theckman/go-flock