luci-mod-network: change ULA explanation

ULA as defined in RFC4193 mentioned a possible future where the L bit
is 0 (implying some global coordinator be used). Locally generated ULA
always have the L bit set to 1, so the prefix is always fd00::/8 (though
ULA with the L bit set 0 or 1 are treated identically).

Technically, fc00::/7 isn't a range, but a prefix which implies a range,
conveniently partitioned by and at the L bit. A range would be correctly
denoted as fc00::-fdff:ffff:ffff:ffff:ffff:ffff:ffff:ffff.

The rewording avoids mention of fc00::/7. The randomly generated prefix
in all cases starts fd.

The sentences were split to ease i18n.

Signed-off-by: Paul Donald <newtwen+github@gmail.com>