add iOS and versions to Safari percent rounding warning; fixes #10723
document that multiple navbars aren't supported by default; addresses #10824
Merge pull request #10826 from twbs/doc-multiple-navbars-unsupported

document that multiple navbars aren't supported by default
Merge pull request #10806 from twbs/ios-safari-pct-rounding

add iOS and versions to Safari percent rounding warning; fixes #10723