Improve X6 parameter description

@vazken.andreassian, friday 18 December 2020 16:06:59

@olivier.delaigue and @charles.perrin,

I don't like the name of the X6 parameter in the airGR help file:

• GR6J X1 production store capacity [mm]
• GR6J X2 intercatchment exchange coefficient [mm/d]
• GR6J X3 routing store capacity [mm]
• GR6J X4 unit hydrograph time constant [d]
• GR6J X5 intercatchment exchange threshold [-]
• GR6J X6 coefficient for emptying exponential store [mm]

I didn't find the name that was given to him in the publications...

Wouldn't "parameter of the exponential store" or "exponential store depletion coefficient" be better?