Improve documentation for new ISTL matrix backend

The new ISTL matrix backend requires users to provide an estimate for the average number of entries per row during GridOperator construction time. Unfortunately, it's not really possible to provide a sensible global default as it depends on the type of space and whether the matrix is blocked or not.

But we should help our users by giving them some formulas for common cases. However, I'm not really sure where to put those recipes. Some options would be:

• the Doxygen documentation of the backend
• the release notes
• a dedicated file somewhere

In any case, the release notes should refer to this collection of defaults.