doc: core.ignoreStat update, and clarify the --assume-unchanged effect
The assume-unchanged bit, and consequently core.ignoreStat, can be
misunderstood. Be assertive about the expectation that file changes should
notified to Git.
Overhaul the general wording thus:
1. direct description of what is ignored given first.
2. example instruction of the user manual action required.
3. use sideways indirection for assume-unchanged and update-index
references.
4. add a 'normally' to give leeway for the change detection.
Signed-off-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>