Make custom max-width on row conform to Boostrap convention

Bootstrap break point for tablet portrait 768 keeps the row intact, so it's not appropriate to have off-canvas elements at that width. Custom off-canvas.css should be breakpoint 767px
Prevent X-scroll on small screens

html overflow-x must be hidden to prevent accidental x-scroll on small screens. This is more apparent with touch screens, where the user often gestures diagonally instead of a true vertical gesture.
Getting started docs - Migrating section: refactor some of the text for clarity and easy reading.Also removed some rogue tab characters in the source that I introduced in prior commits.
CSS docs - Grid section: two dots are missing before 'col-md-8'

Add the two missing docs
Merge pull request #10423 from hustlzp/master

CSS docs - Grid section: two dots are missing before 'col-md-8'
Fixed error in documentation in mixins.less
Merge pull request #10427 from liamzebedee/patch-1

Fixed error in documentation in mixins.less
For consistency with the rest of the docs, removed manually line-wrapped HTML text.
No manual line breaks in HTML text as per custom.
Fixes #10436: use screen-sm var instead of hardcoded pixel value
Merge branch 'master' of github.com:twbs/bootstrap
Fixed to lowercase 'v' in 'v3.0'
Removed a trailing space at end of line.
Yet another  manually wrapped text line consolidated
fixed a </p> tag that was on its own line.
Merge pull request #10421 from StevenBlack/GSEdits6

Getting started docs - Migrating section: refactor the text for ...
Fixes #10417: Document `.hidden` in the Helper classes *Screen reader content* section
Merge pull request #10227 from UTCWebDev/master

Fixes 10212 - Make custom max-width on row conform to Bootstrap convention
Merge branch 'master' of github.com:twbs/bootstrap
polish off #10227 with an inline comment