Make the documentation for increment/decrement clearer

The increment and decrement tags are special in that they would output or echo the variable's final value onto the page (or whatever output sink you're using). This needs to be mentioned in the documentation.

Please see Shopify#1399 for more info and discussion.

_tags/variable.md

@@ -79,7 +79,7 @@ I am 35 and my favourite food is pizza.
 
 ## increment
 
-Creates a new number variable, and increases its value by one every time it is called. The initial value is 0.
+Creates and outputs a new number variable. On subsequent calls, it increases its value by one and outputs the new value. The initial value is 0.
 
 <p class="code-label">Input</p>
 ```liquid
@@ -122,7 +122,7 @@ In the example below, a variable named "var" is created through `assign`. The `i
 
 ## decrement
 
-Creates a new number variable, and decreases its value by one every time it is called. The initial value is -1.
+Creates and outputs a new number variable. On subsequent calls, it decreases its value by one and outputs the new value. The initial value is -1.
 
 <p class="code-label">Input</p>
 ```liquid