Added $ documentation to the manual

It's mentioned for enums, for exmplae, but it should probably be explicit in the string section

doc/manual/types.txt

@@ -342,6 +342,34 @@ builtin ``len`` procedure; the length never counts the terminating zero.
 The assignment operator for strings always copies the string.
 The ``&`` operator concatenates strings.
 
+Most native Nim types support conversion to strings with the special ``$`` proc.
+When calling the ``echo`` proc, for example, the built-in stringify operation
+for the parameter is called:
+
+.. code-block:: nim
+
+  echo 3 # calls `$` for `int`
+
+Whenever a user creates a specialized object, implementation of this procedure
+provides for ``string`` representation.
+
+.. code-block:: nim
+  type
+    Person = object
+      name: string
+      age: int
+
+  proc `$`(p: Person): string = # `$` always returns a string
+    result = p.name & " is " &
+            $p.age & # we *need* the `$` in front of p.age, which
+                     # is natively an integer, to convert it to
+                     # a string
+            " years old."
+
+While ``$p.name`` can also be used, the ``$`` operation on a string does
+nothing. Note that we cannot rely on automatic conversion from an ``int`` to
+a ``string`` like we can for the ``echo`` proc.
+
 Strings are compared by their lexicographical order. All comparison operators
 are available. Strings can be indexed like arrays (lower bound is 0). Unlike
 arrays, they can be used in case statements: