documentation for type signature members

Author: ginsbach

docs/codeql/ql-language-reference/signatures.rst

@@ -40,10 +40,11 @@ Type signatures
 ===============
 
 Type signatures declare module parameters that will be substituted with types when the module is instantiated.
-Type signatures are used to specify supertypes and are the simplest category of signatures.
+Type signatures may specify supertypes and required member predicates (in addition to those member predicates that are
+implied by the supertypes).
 
 The substitution of type signatures relies on structural typing. That is, types do not have to be explicitly defined as
-implementing a type signature - they just need to have the specified (transitive) supertypes.
+implementing a type signature - they just need to have the specified (transitive) supertypes and member predicates.
 
 In detail, a type signature definition consists of:
 
@@ -52,14 +53,19 @@ In detail, a type signature definition consists of:
 #. The name of the type signature. This is an `identifier <https://codeql.github.com/docs/ql-language-reference/ql-language-specification/#identifiers>`_
    starting with a uppercase letter.
 #. Optionally, the keyword ``extends`` followed by a list of types, separated by commas.
-#. A semicolon ``;``.
+#. Either a semicolon ``;`` or a list of predicate signatures enclosed in braces.
+   The ``signature`` keyword is omitted for these contained signatures.
 
 For example:
 
 .. code-block:: ql
 
     signature class ExtendsInt extends int;
 
+    signature class CanBePrinted {
+      string toString();
+    }
+
 Module signatures
 =================