add documention in IOContext docstring

base/arrayshow.jl

@@ -16,8 +16,9 @@
 # The array printing methods here can be grouped into two categories (and are annotated as such):
 # 1) "typeinfo aware" : these are "API boundaries" functions, which will read the typeinfo
 #    property from the context, and pass down to their value an updated property
-#    according to its eltype;
-# 2) "typeinfo agnostic": this are helper functions used by the first category; hence
+#    according to its eltype; at each layer of nesting, only one "typeinfo aware"
+#    function must be called;
+# 2) "typeinfo agnostic": these are helper functions used by the first category; hence
 #    they don't manipulate the typeinfo property, and let the printing routines
 #    for their elements read directly the property set by their callers
 #

base/show.jl

@@ -61,6 +61,11 @@ The following properties are in common use:
  - `:displaysize`: A `Tuple{Int,Int}` giving the size in rows and columns to use for text
    output. This can be used to override the display size for called functions, but to
    get the size of the screen use the `displaysize` function.
+ - `:typeinfo`: a type specification characterizing the information already expressed
+   concerning the type of the object about to be displayed. This is mainly useful when
+   displaying a collection of object of the same type, so that redundant type information
+   can be avoided (e.g. `[[Int16(0)]]` can be shown as "Array{Int16,1}[[0]]" instead
+   of "Array{Int16,1}[Int16[0]]"
 
 ```jldoctest
 julia> function f(io::IO)