'done' -> 'tesla_done'.

This name in tesla-macros.h is *particularly* likely to cause namespace collisions.
Clarify that tesla-macros.h is just a template.

The tesla-macros.h that we distribute is not intended to be a "fait accompli": in its distributed form it may very well cause namespace collisions, so it's best to view it as a template for programmers to adapt to their local needs.