Update docstring of FileProduct (#380)

Author: peremato

src/Products.jl

@@ -440,10 +440,11 @@ function locate(ep::ExecutableProduct, prefix::Prefix; platform::AbstractPlatfor
 end
 
 """
-    FileProduct(path::AbstractString, varname::Symbol, dir_path = nothing)
+    FileProduct(path, varname::Symbol, dir_path = nothing)
 
 Declares a [`FileProduct`](@ref) that points to a file located relative to the root of
-a `Prefix`, must simply exist to be satisfied.
+a `Prefix`, must simply exist to be satisfied. The first argument `path` is either a single `AbstractString` 
+or a `Vector{String}` to allow several paths to be checked.   
 """
 struct FileProduct <: Product
     paths::Vector{String}