doc: notes: clearly state that --stripspace is the default

doc: notes: clearly state that --stripspace is the default

Clearly state when which of the regular and negated form of the
option take effect.[1]

Also mention the subtle behavior that occurs when you mix options like
`-m` and `-C`, including a note that it might be fixed in the future.
The topic was brought up on v8 of the `--separator` series.[2][3]

[1]: https://lore.kernel.org/git/xmqqcyct1mtq.fsf@gitster.g/
[2]: https://lore.kernel.org/git/xmqq4jp326oj.fsf@gitster.g/
† 3: v11 was the version that landed

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>