Even more io doctests (#24928)

Author: kshyatt

base/io.jl

@@ -65,13 +65,45 @@ function readavailable end
     isreadable(io) -> Bool
 
 Return `true` if the specified IO object is readable (if that can be determined).
+
+# Examples
+```jldoctest
+julia> open("myfile.txt", "w") do io
+           write(io, "Hello world!");
+           isreadable(io)
+       end
+false
+
+julia> open("myfile.txt", "r") do io
+           isreadable(io)
+       end
+true
+
+julia> rm("myfile.txt")
+```
 """
 function isreadable end
 
 """
     iswritable(io) -> Bool
 
 Return `true` if the specified IO object is writable (if that can be determined).
+
+# Examples
+```jldoctest
+julia> open("myfile.txt", "w") do io
+           write(io, "Hello world!");
+           iswritable(io)
+       end
+true
+
+julia> open("myfile.txt", "r") do io
+           iswritable(io)
+       end
+false
+
+julia> rm("myfile.txt")
+```
 """
 function iswritable end
 function copy end