Document the cover REST calls (#1195)

Co-authored-by: Franck Nijhof <frenck@frenck.nl>

web-api/index.rst

@@ -6,7 +6,7 @@ Web Server API
     :image: espeasy.png
 
 Since version 1.3, ESPHome includes a built-in web server that can be used to view states
-and send commands. In addition to visible the web-frontend available under the root index of the
+and send commands. In addition to the web-frontend available under the root index of the
 web server, there's also two other features the web server currently offers: A real time event
 source and REST API.
 
@@ -213,3 +213,39 @@ and ``/fan/<id>/toggle``. Turn on additionally supports these optional parameter
 
 -  **speed_level**: The new speed level of the fan. Values as above.
 -  **oscillation**: The new oscillation setting of the fan. Values as above.
+
+Cover
+*****
+
+Covers are again similar to switches whose two possible states are ``OPEN`` and ``CLOSED``.  They
+can however be in an intermediate position, anywhere between **0.0** (fully closed) to **1.0**
+(fully open). They usually take some time to move from one position to another and can also be
+stopped midway. An example GET request for ``/cover/front_window_blinds`` might return:
+
+.. code-block:: json
+
+    {
+      "id": "cover-front_window_blinds",
+      "state": "OPEN",
+      "value": 0.8,
+      "current_operation": "IDLE",
+      "tilt": 0.5
+    }
+
+-  **id**: The ID of the cover, prefixed with ``cover-``.
+-  **state**: ``OPEN`` or ``CLOSED``. Any position other than 0.0 is considered open.
+-  **value**: Current cover position as a float number.
+-  **current_operation**: ``OPENING``, ``CLOSING`` or ``IDLE``.
+-  **tilt**: (only if supported by this cover component) tilt angle from 0.0 to 1.0.
+
+POST requests on the other hand allow performing actions on the cover, the available
+methods being ``open``, ``close``, ``stop`` and ``set``. The following parameters
+can be used:
+
+-  **position**: The target position for a ``set`` call. The ``open`` method implies
+   a target position of 1.0, ``close`` implies a target position of 0.0.
+-  **tilt**: The tilt angle to set, if supported.
+
+Creating a POST request to ``/cover/front_window_blinds/set?position=0.1&tilt=0.3`` will
+start moving the blinds towards an almost completely closed position and a new tilt
+angle.