v1.6.9.12 docs(man): add explanations in PE_Oudin help page

Refs #64

DESCRIPTION

 Package: airGR
 Type: Package
 Title: Suite of GR Hydrological Models for Precipitation-Runoff Modelling
-Version: 1.6.9.11
+Version: 1.6.9.12
 Date: 2021-01-11
 Authors@R: c(
   person("Laurent", "Coron", role = c("aut", "trl"), comment = c(ORCID = "0000-0002-1503-6204")),

NEWS.md

@@ -2,7 +2,7 @@
 
 
 
-### 1.6.9.11 Release Notes (2021-01-11)
+### 1.6.9.12 Release Notes (2021-01-11)
 
 #### New features
 

man/PE_Oudin.Rd

@@ -48,6 +48,8 @@ Function which computes PE using the formula from Oudin et al. (2005). PE can be
 }
 
 \details{
+To calculate basin-wide Oudin potential evapotranspiration, it is advised, when possible, to use either station temperature or gridded temperature data to calculate PE and then average these PE values at the basin scale.
+
 In the \code{JD} argument, the Julian day of the year of the 1st of January is equal to 1 and the 31st of December to 365 (366 in leap years). If the Julian day of the year is computed on an object of the \code{POSIXlt} class, the user has to add 1 to the returned value (e.g. \code{as.POSIXlt("2016-12-31")$yday + 1}).
 
 When hourly temperature is provided, all the values of the same day have to be set to the same Julian day of the year (e.g. \code{as.POSIXlt("2016-12-31 00:00:00")$yday + 1} and \code{as.POSIXlt("2016-12-31 00:01:00")$yday + 1}). Each single day must be provided 24 identical Julian day values (one for each hour).