Fix documentation for grouping

Incorrect location of comments for class meant that the documentation
for this class was not been shown.

Also made minor change to the documentation of ````orderFunction````.
Add link in documentation to Array.prototype.sort() on MDN

Our documentation for insight.Grouping refers to Array.sort(), so
should link to online documentation for that function.
Merge remote-tracking branch 'scottlogic/develop' into feature/fix-grouping-documentation
Merge pull request #214 from matthewdunsdon/feature/fix-grouping-documentation

Fix documentation for insight.Grouping