Fixed typos and format inconsistencies (#1123)

Please check these sentences:
"We test the configuration to ensure that users have a great experience and minimize notifications if something is wrong with a platform or component setup before Home Assistant runs."
"If a sensor has a pre-defined list of available options, test to make sure the configuration entry matches the list."

I wasn't sure if I got the intent correct, and in the second one I wasn't sure what you meant by "it"

source/developers/development_validation.markdown

@@ -9,11 +9,11 @@ sharing: true
 footer: true
 ---
 
-The `configuration.yaml` file contains the configuration options for components and platforms. To ensure that the given configuration provided by the user is valid we use [voluptuous](https://pypi.python.org/pypi/voluptuous) to check it. Certain entries are optional or could be required for the setup of a platform or a component. Others must be of a definied type or out of an already defined list.
+The `configuration.yaml` file contains the configuration options for components and platforms. We use [voluptuous](https://pypi.python.org/pypi/voluptuous) to make sure that the configuration provided by the user is valid. Some entries are optional or could be required to set up a platform or a component. Others must be a defined type or from an already-defined list.
 
-The goal of testing the configuration is to assure that users have a great experience due to notifications if something is wrong with a platform or component setup before Home Assistant is running.
+We test the configuration to ensure that users have a great experience and minimize notifications if something is wrong with a platform or component setup before Home Assistant runs.
 
-Beside the [voluptuous](https://pypi.python.org/pypi/voluptuous) default types are a bunch of custom types available. To get a full overview take a look at the [config_validation.py](https://github.com/home-assistant/home-assistant/blob/master/homeassistant/helpers/config_validation.py) helper.
+Besides [voluptuous](https://pypi.python.org/pypi/voluptuous) default types, many custom types are available. For an overview, take a look at the [config_validation.py](https://github.com/home-assistant/home-assistant/blob/master/homeassistant/helpers/config_validation.py) helper.
 
 - Types: `string`, `byte`, and `boolean`
 - Entity ID: `entity_id` and `entity_ids`
@@ -21,21 +21,21 @@ Beside the [voluptuous](https://pypi.python.org/pypi/voluptuous) default types a
 - Time: `time`, `time_zone`
 - Misc: `template`, `slug`, `temperature_unit`, `latitude`, `longitude`, `isfile`, `sun_event`, `ensure_list`, `port`, `url`,  and `icon`
  
-To validate plaforms using [MQTT](/components/mqtt/) there are `valid_subscribe_topic` and `valid_publish_topic` present.
+To validate plaforms using [MQTT](/components/mqtt/), `valid_subscribe_topic` and `valid_publish_topic` are available.
 
 Some things to keep in mind:
 
-- Use the constants which are definded in `const.py`.
+- Use the constants defined in `const.py`
-- Import `PLATFORM_SCHEMA` from parent component and extend it.
+- Import `PLATFORM_SCHEMA` from the parent component and extend it
-- Preferred order is `required` first, then `optional`.
+- Preferred order is `required` first and `optional` second
 
 ### {% linkable_title Snippets %} 
 
-This section contains a couple of snippets for the validation we use.
+This section contains snippets for the validation we use.
 
-### {% linkable_title Default name %} 
+#### {% linkable_title Default name %} 
 
-It's common to set a default for a sensor if the user is not providing a name to use.
+It's common to set a default for a sensor if the user doesn't provide a name to use.
 
 ```python
 DEFAULT_NAME = 'Sensor name'
@@ -45,9 +45,9 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Optional(CONF_NAME, default=DEFAULT_NAME): cv.string,
 ```
 
-### {% linkable_title Limit the values %} 
+#### {% linkable_title Limit the values %} 
 
-In certain cases you want to limit the user's input to a couple of options.
+You might want to limit the user's input to a couple of options.
 
 ```python
 DEFAULT_METHOD = 'GET'
@@ -57,9 +57,9 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Optional(CONF_METHOD, default=DEFAULT_METHOD): vol.In(['POST', 'GET']),
 ```
 
-### {% linkable_title Port %} 
+#### {% linkable_title Port %} 
 
-As all port numbers are coming out of the range 1 till 65535. 
+All port numbers are from a range of 1 to 65535. 
 
 ```python
 DEFAULT_PORT = 993
@@ -69,9 +69,9 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Optional(CONF_PORT, default=DEFAULT_PORT): cv.port,
 ```
 
-### {% linkable_title Lists %} 
+#### {% linkable_title Lists %} 
 
-If a sensor has a pre-defined list of available options it should be tested if the configuration entry matches it.
+If a sensor has a pre-defined list of available options, test to make sure the configuration entry matches the list.
 
 ```python
 SENSOR_TYPES = {
@@ -84,5 +84,3 @@ PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Optional(CONF_MONITORED_VARIABLES, default=[]):
         vol.All(ensure_list, [vol.In(SENSOR_TYPES)]),
 ```
-
-