Complete documentation in more demos

[Martin Sandve Alnæs]

Some of the documented demos only have e.g. a python version, missing a cpp version.

Many of the undocumented demos would of course be nice to have documented.

Created this issue to represent this work. Feel free to suggest particular demos that would be nice to have documented better.

[Anders Logg (Chalmers)]

Removing milestone: 1.8 (automated comment)