Add docs for the different argument types as well as for new inline image support.

docs/extensions.md

@@ -119,8 +119,50 @@ class SomeBlocks {
     // ...
 }
 ```
+### Block Arguments
+In addition to displaying text, blocks can have arguments in the form of slots to take other blocks getting plugged in, or dropdown menus to select an argument value from a list of possible values.
 
-### Defining a Menu
+The possible types of block arguments are as follows:
+
+- String - a string input, this is a type-able field which also accepts other reporter blocks to be plugged in
+- Number - an input similar to the string input, but the type-able values are constrained to numbers.
+- Angle - an input similar to the number input, but it has an additional UI to be able to pick an angle from a
+circular dial
+- Boolean - an input for a boolean (hexagonal shaped) reporter block. This field is not type-able.
+- Color - an input which displays a color swatch. This field has additional UI to pick a color by choosing values for the color's hue, saturation and brightness.
+- Matrix - an input which displays a 5 x 5 matrix of cells, where each cell can be filled in or clear.
+- Note - a numeric input which can select a musical note. This field has additional UI to select a note from a
+visual keyboard.
+- Image - an inline image displayed on a block. This is a special argument type in that it does not represent a value and does not accept other blocks to be plugged-in in place of this block field. See the section below about "Adding an Inline Image".
+
+#### Adding an Inline Image
+In addition to specifying block arguments (an example of string arguments shown in the code snippet above),
+you can also specify an inline image for the block. You must include a dataURI and alt text for the image.
+You can optionally also specify `flipRTL`, a property indicating whether the image should be flipped horizontally when the editor has a right to left language selected as its locale. By default, the image is not flipped.
+
+```js
+return {
+    // ...
+    blocks: [
+        {
+            //...
+            arguments {
+                MY_IMAGE: {
+                    type: ArgumentType.IMAGE,
+                    dataURI: 'myImageData',
+                    alt: 'This is an image',
+                    flipRTL: true
+                }
+            }
+        }
+    ]
+}
+```
+
+
+
+
+#### Defining a Menu
 
 To display a drop-down menu for a block argument, specify the `menu` property of that argument and a matching item in
 the `menus` section of your extension's definition:
@@ -201,7 +243,7 @@ menus: {
 }
 ```
 
-#### Accepting reporters ("droppable" menus)
+##### Accepting reporters ("droppable" menus)
 
 By default it is not possible to specify the value of a dropdown menu by inserting a reporter block. While we
 encourage extension authors to make their menus accept reporters when possible, doing so requires careful