clarify the meaning of the 'auto' metatype; 'auto' is now bind-multiple; fixes #3224

doc/manual/generics.txt

@@ -116,7 +116,7 @@ type class           matches
 ``array``            any array type
 ``set``              any set type
 ``seq``              any seq type
-``auto``             any type
+``any``              any type
 ==================   ===================================================
 
 Furthermore, every generic type automatically creates a type class of the same
@@ -163,15 +163,6 @@ module to illustrate this:
 Alternatively, the ``distinct`` type modifier can be applied to the type class
 to allow each param matching the type class to bind to a different type.
 
-If a proc param doesn't have a type specified, Nim will use the
-``distinct auto`` type class (also known as ``any``). Note this behavior is
-deprecated for procs; templates, however, support them:
-
-.. code-block:: nim
-  # allow any combination of param types
-  proc concat(a, b): string = $a & $b # deprecated
-  proc concat(a, b: any): string = $a & $b # preferred
-
 Procs written with the implicitly generic style will often need to refer to the
 type parameters of the matched generic type. They can be easily accessed using
 the dot syntax:

doc/manual/types.txt

@@ -1228,3 +1228,27 @@ However, a ``void`` type cannot be inferred in generic code:
 
 The ``void`` type is only valid for parameters and return types; other symbols
 cannot have the type ``void``.
+
+
+Auto type
+---------
+
+The ``auto`` type can only be used for return types and parameters. For return
+types it causes the compiler to infer the type from the routine body:
+
+.. code-block:: nim
+  proc returnsInt(): auto = 1984
+
+For parameters it currently creates implicitly generic routines:
+
+.. code-block:: nim
+  proc foo(a, b: auto) = discard
+
+Is the same as:
+
+.. code-block:: nim
+  proc foo[T1, T2](a: T1, b: T2) = discard
+
+However later versions of the language might change this to mean "infer the
+parameters' types from the body". Then the above ``foo`` would be rejected as
+the parameters' types can not be infered from an empty ``discard`` statement.