Update docs on conditional syntax

SYNTAX.md

@@ -6,16 +6,18 @@ Documents the template syntax.
 
 ## Table of Contents
 
-- [Expressions](#expressions)
-  - [Literals](#literals)
-  - [Values](#values)
-  - [Filters](#filters)
-- [Blocks](#blocks)
-  - [Conditionals](#conditionals)
-  - [Loops](#loops)
-  - [With](#with)
-  - [Include](#include)
-- [Whitespace control](#whitespace-control)
+- [syntax](#syntax)
+  - [Table of Contents](#table-of-contents)
+  - [Expressions](#expressions)
+    - [Literals](#literals)
+    - [Values](#values)
+    - [Filters](#filters)
+  - [Blocks](#blocks)
+    - [Conditionals](#conditionals)
+    - [Loops](#loops)
+    - [With](#with)
+    - [Include](#include)
+  - [Whitespace control](#whitespace-control)
 
 
 An `upon` template is simply a piece of UTF-8 text. It can be embedded in
@@ -103,8 +105,10 @@ Conditionals are marked using an opening `if` block and a closing `endif`
 block. It can also have zero or more optional `else if` clauses and an
 optional `else` clause. A conditional renders the contents of the block
 based on the specified condition which can be any
-[**expression**](#expressions) but it must resolve to a boolean value.  A
-boolean expression can be negated by applying the prefix `not`.
+[**expression**](#expressions). None, `false`, zero floats and integers,
+as well as empty strings, maps, and lists are considered falsy. Everything
+else is considered truthy. A boolean expression can be negated by applying
+the prefix `not`.
 
 Consider the following template. If the nested field `user.is_enabled` is
 returns `false` then the first paragraph would be rendered. Otherwise if