rearrange, add comments

scss/_functions.scss

+// Bootstrap functions
+//
+// Utility mixins and functions for evalutating source code across our variables, maps, and mixins.
+
+// Ascending
+// Used to evaluate Sass maps like our grid breakpoints.
 @mixin _assert-ascending($map, $map-name) {
   $prev-key: null;
   $prev-num: null;
@@ -14,7 +20,19 @@
   }
 }
 
+// Starts at zero
+// Another grid mixin that ensures the min-width of the lowest breakpoint starts at 0.
+@mixin _assert-starts-at-zero($map) {
+  $values: map-values($map);
+  $first-value: nth($values, 1);
+  @if $first-value != 0 {
+    @warn "First breakpoint in `$grid-breakpoints` must start at 0, but starts at #{$first-value}.";
+  }
+}
+
 // Replace `$search` with `$replace` in `$string`
+// Used on our SVG icon backgrounds for custom forms.
+//
 // @author Hugo Giraudel
 // @param {String} $string - Initial string
 // @param {String} $search - Substring to replace
@@ -29,11 +47,3 @@
 
   @return $string;
 }
-
-@mixin _assert-starts-at-zero($map) {
-  $values: map-values($map);
-  $first-value: nth($values, 1);
-  @if $first-value != 0 {
-    @warn "First breakpoint in `$grid-breakpoints` must start at 0, but starts at #{$first-value}.";
-  }
-}