Fixes 10212 - Make custom max-width on row conform to Bootstrap convention. polish off 10227 with an inline comment

BS3 commits: 88e1e20b95f573a0ab8ef2a60d00140a7b329d15,
c0f6d722b02c8dcf167cc5f55fcfbadaf34d58a2
Fixes 10417: Document `.hidden` in the Helper classes *Screen reader content* section

BS3 commits: 6ce9b4d0419b1afbea11afe65a47c82966a89a2a