Document include path separator normalization via normpath (#42429)

Author: mkitti

base/client.jl

@@ -472,6 +472,8 @@ Returns the result of the last evaluated expression of the input file. During in
 a task-local include path is set to the directory containing the file. Nested calls to
 `include` will search relative to that path. This function is typically used to load source
 interactively, or to combine files in packages that are broken into multiple source files.
+The argument `path` is normalized using [`normpath`](@ref) which will resolve
+relative path tokens such as `..` and convert `/` to the appropriate path separator.
 
 The optional first argument `mapexpr` can be used to transform the included code before
 it is evaluated: for each parsed expression `expr` in `path`, the `include` function

base/path.jl

@@ -356,12 +356,16 @@ joinpath
 """
     normpath(path::AbstractString) -> String
 
-Normalize a path, removing "." and ".." entries.
+Normalize a path, removing "." and ".." entries and changing "/" to the canonical path separator
+for the system.
 
 # Examples
 ```jldoctest
 julia> normpath("/home/myuser/../example.jl")
 "/home/example.jl"
+
+julia> normpath("Documents/Julia") == joinpath("Documents", "Julia")
+true
 ```
 """
 function normpath(path::String)