doc(XCP-NG): place SR on ZFS dataset instead of pool itself

[p-bo]

Hello,
Storage Repository should be placed on ZFS dataset, not ZFS pool itself.

• if no dataset present, zfs kernel module will be not loaded automatically - see this comment
• summary - see this comment
• it is even consistent with section "Better performance (advanced options)" below this part

Hope it will save others some confusion :-)

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[1] https://creativecommons.org/licenses/by-sa/2.0/
[2] https://docs.xcp-ng.org/project/contributing/#developer-certificate-of-origin-dco

[p-bo]

@thomas-dkmt May I ask you to review (and eventually merge) please? Thanks :-)