Mention how to use list variables with `Get Length`.
Mention how to use list variables with `Get Length`.

Update issue 850
Summary: Document how to use list variables with length related keywords.
Status: Done
Owner: pekka.klarck
Labels: Type-Documentation Priority-Low Target-2.6
It's true that the library documentation didn't explain this at all.
I decided to update the documentation of `Get Length` instead of
`Should Not Be Empty` because all length related keywords use it
internally and have link pointing to it already.

waldenner/robotframework,fiuba08/robotframework,waldenner/robotframework,fiuba08/robotframework,waldenner/robotframework,ldtri0209/robotframework,waldenner/robotframework,ldtri0209/robotframework,fiuba08/robotframework,fiuba08/robotframework,ldtri0209/robotframework,waldenner/robotframework,ldtri0209/robotframework,fiuba08/robotframework,ldtri0209/robotframework