Add note about page vs post. [ci skip]

Closes #3056.

site/_docs/posts.md

@@ -134,6 +134,11 @@ Of course, you have full control over how (and where) you display your posts,
 and how you structure your site. You should read more about [how templates
 work](../templates/) with Jekyll if you want to know more.
 
+Note that the `post` variable only exists inside the `for` loop above. If
+you wish to access the currently-rendering page/posts's variables (the
+variables of the post/page that has the `for` loop in it), use the `page`
+variable instead.
+
 ## Post excerpts
 
 Each post automatically takes the first block of text, from the beginning of the content