Chrome uses XS grid when printing for US Letter or A4 paper

[cvrebert]

See twbs/bootstrap#12078
ResponsiveBP's grid is affected in the same way.
This is probably unintuitive enough to warrant being mentioned in the docs.
(Also possibly applies to IE11, but I don't have a Windows box to test with.)

[JimBobSquarePants]

Thanks for that. I'll get the documentation updated.

[JimBobSquarePants]

On further investigation I've found that by shifting the breakpoints using print only media queries I can lower the threshold for the xs to s grid level transition. That ensures that the s level grid will render on A4 and US letter pages when rendered with the default print margins whilst also respecting visibility helpers.

I'll reference here on the next commit.