RMSEA: add correction note

Author: alyst

src/frontend/fit/fitmeasures/RMSEA.jl

@@ -13,6 +13,11 @@ for the SEM model.
 
 # See also
 [`fit_measures`](@ref), [`χ²`](@ref), [`dof`](@ref)
+
+# Extended help
+
+For multigroup models, the correction proposed by J.H. Steiger is applied
+(see [Steiger, J. H. (1998). *A note on multiple sample extensions of the RMSEA fit index*](https://doi.org/10.1080/10705519809540115)).
 """
 function RMSEA end