add doc to define function

lib/postgrex/type_module.ex

@@ -3,6 +3,50 @@ defmodule Postgrex.TypeModule do
 
   alias Postgrex.TypeInfo
 
+  @doc """
+  Creates a module to aid in type processing.
+
+  The resulting module has three main purposes:
+
+      1. Associate the type information from the bootstrap query to extensions
+      2. Encode Elixir data types into binaries to send to the Postgres server
+      3. Decode binaries sent from the Postgres server into Elixir data types
+
+  The first point is handled by creating a `find/2` function that accepts a `%Postgrex.TypeInfo{}`
+  struct and a format such as `:binary`, `:text`, `:any`. It returns either a 2-tuple for
+  regular extensions containing the format and the extension name or a 3-tuple for super
+  extensions containing the format, the extension name and the sub-oids. Some important points:
+
+      - The type infos are associated to extensions using the `matching/1` callback
+      - The sub-oids for super extensions are created using the `oids/2` callback
+
+  The second point is handled by creating encoding functions for each extension. These functions
+  are created by iterating over each extension, taking the pattern returned by the `encode/1`
+  callback and creating a function that either:
+
+      - Accepts the pattern for regular extensions
+      - Accepts the pattern, sub-oids and sub-tupes for super extensions
+
+  Each function is given the same name as the extension module and simply returns the body of the
+  `encode/1` callback. Several helper functions that help encode parameters, lists, tuples and
+  values are also exposed and can be called either locally or remotely. These helper functions
+  eventually call the encoding functions specific to each extension.
+
+  The third point is handled by creating decoding functions for each extension. These functions
+  are created by iterating over each extension, taking the pattern returned by the `decode/1`
+  callback and creating a function that either:
+
+      - Accepts the pattern for regular extensions
+      - Accepts the pattern, sub-oids and sub-tupes for super extensions
+
+  Each function also accepts several variables that help accumulate the results and a special
+  variable `mod` that allows the extensions to access the type modifier of the column. This
+  allows them, for example, to return the correct precision for timestamps.
+
+  Each function is given the same name as the extension module and adds the body of the `decode/1`
+  callback to the accumulated results. Helper functions that help decode lists and tuples are also
+  exposed. These helper functions eventually call the decoding functions specific to each extension.
+  """
   def define(module, extensions, opts) do
     opts =
       opts