docs: Add hashes to for tag documentation

This extremely helpful information was found at https://github.com/Shopify/liquid/wiki/Liquid-for-Designers#allowed-collection-types

docs/source/tags/for.md

@@ -8,7 +8,7 @@ Iteration tags run blocks of code repeatedly.
 
 ### for...in
 
-Repeatedly executes a block of code. For a full list of attributes available within a `for` loop.
+Repeatedly executes a block of code. For a full list of attributes available within a `for` loop, see [forloop](#forloop).
 
 Input
 ```liquid
@@ -22,6 +22,23 @@ Output
 hat shirt pants
 ```
 
+For loops can iterate over arrays, hashes, and [ranges of integers](#range).
+
+When iterating a hash, item[0] contains the key, and item[1] contains the value:
+
+Input
+```liquid
+{% for item in hash %}
+  * {{ item[0] }}: {{ item[1] }}
+{% endfor %}
+```
+
+Output
+```text
+  * key1: value1
+  * key2: value2
+```
+
 ### else
 
 Specifies a fallback case for a `for` loop which will run if the loop has zero length.