add documentation for extensible annotation

Author: ginsbach

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

@@ -704,6 +704,7 @@ Various kinds of syntax can have *annotations* applied to them. Annotations are
    simpleAnnotation ::= "abstract"
                     |   "cached"
                     |   "external"
+                    |   "extensible"
                     |   "final"
                     |   "transient"
                     |   "library"
@@ -735,6 +736,8 @@ The following table summarizes the syntactic constructs which can be marked with
 +----------------+---------+------------+-------------------+-----------------------+---------+--------+---------+---------+
 | ``external``   |         |            |                   | yes                   |         |        |         |         |
 +----------------+---------+------------+-------------------+-----------------------+---------+--------+---------+---------+
+| ``extensible`` |         |            |                   | yes                   |         |        |         |         |
++----------------+---------+------------+-------------------+-----------------------+---------+--------+---------+---------+
 | ``final``      | yes     |            | yes               |                       |         | yes    |         | yes     |
 +----------------+---------+------------+-------------------+-----------------------+---------+--------+---------+---------+
 | ``transient``  |         |            |                   | yes                   |         |        |         |         |
@@ -2162,6 +2165,7 @@ The complete grammar for QL is as follows:
    simpleAnnotation ::= "abstract"
                     |   "cached"
                     |   "external"
+                    |   "extensible"
                     |   "final"
                     |   "transient"
                     |   "library"