Explicitly explain what epsilon is and how it relates to strain/strain rate

[bangerth]

Following a recent discussion about this, it occurred to me that it would be useful to explicitly say that \varepsilon is an operator, not something that stands on its own.

[gassmoeller]

Looks good. The cookbook for finite strain now uses a slightly different notation, but it explains it explicitly, and this is still useful for the rest of the manual (and all the documentation in the code).