Clarify derivatives.

[HansOlsson]

Closes #3846

[HansOlsson]

Pure clarification, no actual change.

[henrikt-ma]

As a way to remedy the lack of a way to express infinite differentiability, perhaps the specification should recommend a specific big value to approximate infinity?!

[henrikt-ma]

Looks good to me, but it would also be good to get an approval from @AHaumer.

[HansOlsson]

As a way to remedy the lack of a way to express infinite differentiability, perhaps the specification should recommend a specific big value to approximate infinity?!

Good idea, I changed so that 100 is recommended. Basically it is easy to recognize (as any power of 10 would be), and I found 10 to be too small (I could imagine that someone thought it meant exactly 10 derivatives in some weird case), and 1000 excessive.

and pinging @AHaumer