Docs: add grid-row-columns variable

[hadouin]

#36691

[mdo]

This new variable needs to be put to use in the row-cols mixin. See

bootstrap/scss/mixins/_grid.scss

Line 56 in 6cf7253

@mixin row-cols($count) {

.

[hadouin]

@mdo It's already used here, if I'm not mistaken.

bootstrap/scss/mixins/_grid.scss

Lines 82 to 88 in 6f10bdc

	@if $grid-row-columns > 0 {
	@for $i from 1 through $grid-row-columns {
	.row-cols#{$infix}-#{$i} {
	@include row-cols($i);
	}
	}
	}