Resolve Duplicate Docs Warnings (#180)

Warnings were being issued regarding duplicate documentation
declarations. Here's an example:

```
warning: redefining @doc attribute previously set at line 27.

Please remove the duplicate docs. If instead you want to override a previously defined @doc, attach the @doc attribute to a function head (the function signature not followed by any do-block). For example:

    @doc """
    new docs
    """
    def use_cassette(...)

  lib/exvcr/mock.ex:48: ExVCR.Mock.use_cassette/3
```

This change resolves these warnings by merging documentation and adding
examples that were already available in the README file.

lib/exvcr/config.ex

@@ -19,45 +19,83 @@ defmodule ExVCR.Config do
   @doc """
   Replace the specified pattern with placeholder.
   It can be used to remove sensitive data from the casette file.
+
+  ## Examples
+
+      test "replace sensitive data" do
+        ExVCR.Config.filter_sensitive_data("<PASSWORD>.+</PASSWORD>", "PLACEHOLDER")
+
+        use_cassette "sensitive_data" do
+          assert HTTPotion.get("http://something.example.com", []).body =~ ~r/PLACEHOLDER/
+        end
+
+        # Now clear the previous filter
+        ExVCR.Config.filter_sensitive_data(nil)
+      end
   """
   def filter_sensitive_data(pattern, placeholder) do
     Setting.append(:filter_sensitive_data, {pattern, placeholder})
   end
 
-
-  @doc """
-  Clear the previously specified filter_sensitive_data lists.
-  """
   def filter_sensitive_data(nil) do
     Setting.set(:filter_sensitive_data, [])
   end
 
   @doc """
-  Clear the previously specified filter_request_headers lists.
+  This function can be used to filter headers from saved requests.
+
+  ## Examples
+
+      test "replace sensitive data in request header" do
+        ExVCR.Config.filter_request_headers("X-My-Secret-Token")
+
+        use_cassette "sensitive_data_in_request_header" do
+          body = HTTPoison.get!("http://localhost:34000/server?", ["X-My-Secret-Token": "my-secret-token"]).body
+          assert body == "test_response"
+        end
+
+        # The recorded cassette should contain replaced data.
+        cassette = File.read!("sensitive_data_in_request_header.json")
+        assert cassette =~ "\"X-My-Secret-Token\": \"***\""
+        refute cassette =~  "\"X-My-Secret-Token\": \"my-secret-token\""
+
+        # Now reset the filter
+        ExVCR.Config.filter_request_headers(nil)
+      end
   """
   def filter_request_headers(nil) do
     Setting.set(:filter_request_headers, [])
   end
 
-  @doc """
-  Replace the specified request header with placeholder.
-  It can be used to remove sensitive data from the casette file.
-  """
   def filter_request_headers(header) do
     Setting.append(:filter_request_headers, header)
   end
 
   @doc """
-  Clear the previously specified filter_request_options lists.
+  This function can be used to filter options.
+
+  ## Examples
+
+      test "replace sensitive data in request options" do
+        ExVCR.Config.filter_request_options("basic_auth")
+        use_cassette "sensitive_data_in_request_options" do
+          body = HTTPoison.get!(@url, [], [hackney: [basic_auth: {"username", "password"}]]).body
+          assert body == "test_response"
+        end
+
+        # The recorded cassette should contain replaced data.
+        cassette = File.read!("sensitive_data_in_request_options.json")
+        assert cassette =~ "\"basic_auth\": \"***\""
+        refute cassette =~  "\"basic_auth\": {\"username\", \"password\"}"
+
+        # Now reset the filter
+        ExVCR.Config.filter_request_options(nil)
+      end
   """
   def filter_request_options(nil) do
     Setting.set(:filter_request_options, [])
   end
 
-  @doc """
-  Replace the specified request header with placeholder.
-  It can be used to remove sensitive data from the casette file.
-  """
   def filter_request_options(header) do
     Setting.append(:filter_request_options, header)
   end

lib/exvcr/mock.ex

@@ -25,7 +25,14 @@ defmodule ExVCR.Mock do
   end
 
   @doc """
-  Provides macro to mock response based on specified parameters.
+  Provides macro to trigger recording/replaying http interactions.
+
+  ## Options
+
+  - `:match_requests_on` A list of request properties to match on when
+    finding a matching response. Valid values include `:query`, `:headers`,
+    and `:request_body`
+
   """
   defmacro use_cassette(:stub, options, test) do
     quote do
@@ -45,16 +52,6 @@ defmodule ExVCR.Mock do
     end
   end
 
-  @doc """
-  Provides macro to trigger recording/replaying http interactions.
-
-  ## Options
-
-  - `:match_requests_on` A list of request properties to match on when
-    finding a matching response. Valid values include `:query`, `:headers`,
-    and `:request_body`
-
-  """
   defmacro use_cassette(fixture, options, test) do
     quote do
       recorder = Recorder.start(
@@ -77,9 +74,6 @@ defmodule ExVCR.Mock do
     end
   end
 
-  @doc """
-  Provides macro to trigger recording/replaying http interactions with default options.
-  """
   defmacro use_cassette(fixture, test) do
     quote do
       use_cassette(unquote(fixture), [], unquote(test))