xbps-src: expand note about -t flag.

[ericonr]

Related to #24314

[q66]

afaik there is work in progress to support renameat2 in zfs, so this might not be true after a while

[ericonr]

Made it more generic, then.

[q66]

the wording makes it sound like they are two separate things, i'd reword it it a bit, something like "This flag requires xbps-uchroot(1), so it won't work..."

[ericonr]

I'm not sure it's fully related... If you set XBPS_CHROOT_CMD=uchroot in etc/conf, things still work just fine, unless you also add -t to the xbps-src command line. So it's not because you're using xbps-uchroot, it's because you're using overlayfs.

[q66]

hm, you're right i guess...