document that multiple navbars aren't supported by default; addresses 10824

BS3 commits: 5528c6501cd0f382289630665fa77394146ea68c
add maintenance note regarding 10864's addition. add iOS and versions to Safari percent rounding warning; fixes 10723

BS3 commits: a97918297e0fe8253b4ad9f8a63a1186eae979c4,
f9dd7466cfbc89c2ed458ec83cd29db7aa83525a