Use "Skip to main content" phrasing consistently; further 10786 fixes

BS3 commits: af4d1027671b278bdc3b0069bb7ef871bf936109
Use "Skip to main content" phrasing consistently; further 10786 fixes. add cross-reference to Input Groups in CSS Form docs; fixes 10792

BS3 commits: af4d1027671b278bdc3b0069bb7ef871bf936109,
2fa03734a316e00cab283ab99d8a9e020cf9bd07