doc: core.ignoreStat update, and clarify the --assume-unchanged effect
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 <a0e80625db756b4bd51d5507b8662a7e4f6837ad@iee.org>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git