Documentation differentiation

[CohenCyril]

One should add documentation:

• about the use of and differences between derivable, differentiable, 'D_v f, f^`(), 'd f, is_diff and is_derive
• for the use of is_diff and is_derive in derive.v to perform automated differentiation
  cf #math-comp users > how to build a {linear _ -> _} @ 💬