Update typespecs-and-behaviours.markdown (elixir-lang#1348)

Author: fireproofsocks

getting-started/typespecs-and-behaviours.markdown

@@ -26,7 +26,9 @@ The syntax is to put the function and its input on the left side of the `::` and
 
 In code, function specs are written with the `@spec` attribute, typically placed immediately before the function definition. Specs can describe both public and private functions. The function name and the number of arguments used in the `@spec` attribute must match the function it describes.
 
-Elixir supports compound types as well. For example, a list of integers has type `[integer]`. You can see all the built-in types provided by Elixir [in the typespecs docs](https://hexdocs.pm/elixir/typespecs.html).
+Elixir supports compound types as well. For example, a list of integers has type `[integer]`, or maps that define keys and types (see the example below).
+
+You can see all the built-in types provided by Elixir [in the typespecs docs](https://hexdocs.pm/elixir/typespecs.html).
 
 ### Defining custom types
 
@@ -48,6 +50,17 @@ end
 
 The `@typedoc` attribute, similar to the `@doc` and `@moduledoc` attributes, is used to document custom types.
 
+You may define compound custom types, e.g. maps:
+
+```elixir
+@type error_map :: %{
+   message: String.t,
+   line_number: integer
+}
+```
+
+[Structs](https://elixir-lang.org/getting-started/structs.html) offer similar functionalty.
+
 Let's look at another example to understand how to define more complex types. Say we have a `LousyCalculator` module, which performs the usual arithmetic operations (sum, product, and so on) but, instead of returning numbers, it returns tuples with the result of an operation as the first element and a random remark as the second element.
 
 ```elixir
@@ -91,7 +104,7 @@ defmodule QuietCalculator do
 end
 ```
 
-If you want to keep a custom type private, you can use the `@typep` attribute instead of `@type`. The visibility also affects whether or not documentation will be generated by toolks like [ExDoc](https://hexdocs.pm/ex_doc/readme.html), Elixir's documentation generator.
+If you want to keep a custom type private, you can use the `@typep` attribute instead of `@type`. The visibility also affects whether or not documentation will be generated by tools like [ExDoc](https://hexdocs.pm/ex_doc/readme.html), Elixir's documentation generator.
 
 ### Static code analysis