reference: Move "unit" type to Tuple types

Author: Ulrik Sverdrup

src/doc/reference.md

@@ -3383,17 +3383,10 @@ User-defined types have limited capabilities.
 
 The primitive types are the following:
 
-* The "unit" type `()`, having the single "unit" value `()` (occasionally called
-  "nil"). [^unittype]
 * The boolean type `bool` with values `true` and `false`.
 * The machine types.
 * The machine-dependent integer and floating-point types.
 
-[^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
-    reference variables; the "unit" type is the implicit return type from functions
-    otherwise lacking a return type, and can be used in other contexts (such as
-    message-sending or type-parametric code) as a zero-size type.]
-
 #### Machine types
 
 The machine types are the following:
@@ -3444,6 +3437,14 @@ of the tuple. It has no nominal name and is instead structurally typed.
 Tuple types and values are denoted by listing the types or values of their
 elements, respectively, in a parenthesized, comma-separated list.
 
+The empty tuple is the "unit" type `()`, having the single value `()`
+(occasionally called "nil"). [^unittype]
+
+[^unittype]: The "unit" value `()` is *not* a sentinel "null pointer" value for
+    reference variables; the "unit" type is the implicit return type from functions
+    otherwise lacking a return type, and can be used in other contexts (such as
+    message-sending or type-parametric code) as a zero-size type.]
+
 Because tuple elements don't have a name, they can only be accessed by
 pattern-matching or by using `N` directly as a field to access the
 `N`th element.