bug#20767: seq invocation limitations documentation

[積丹尼 Dan Jacobson]

On (info "(coreutils) seq invocation") perhaps mention if one needs to
use two % items, a for loop might be required,

$ for i in `seq 14484 10000 34484`; do printf %d=0x%x\\n $i $i; done
14484=0x3894
24484=0x5fa4
34484=0x86b4