Resolve "Vérificateur: ajouter un avertissement à l'utilisation"

Closes #643 (closed)

docs/en/calculators/verif/principe.md

 # Crossability verification
 
+## Warning
+
+The purpose of this tool is to carry out a simple check of the compatibility
+between certain fish pass sizing criteria and the passage capacities of the
+target fish species. 
+It may prove useful during the design phase of a facility to ensure that the
+proposed design is consistent with its hydrological operating range.
+
+However, this tool is not sufficient to fully verify the functionality of a
+planned or existing system, insofar as other important dimensional and
+hydraulic criteria are not taken into account by this module 
+(feed flow rate and characteristics of the fish intake in terms of attractiveness,
+and depending on the type of system: basin aspect ratio,
+concentration of macro-roughness, bottom roughness, size of baffles, etc.).
+
 This module allows to verify the capacity of different fish species to cross the following types of fish passes:
 
 - [fish ladders](pab.md)