C++: Add a QL doc.

Author: michaelnebel

cpp/ql/lib/semmle/code/cpp/security/boostorg/asio/protocols.qll

@@ -191,6 +191,9 @@ module BoostorgAsio {
   class SslContextClass extends Class {
     SslContextClass() { this.getQualifiedName() = "boost::asio::ssl::context" }
 
+    /**
+     * Gets a constructor call, if any.
+     */
     ConstructorCall getAConstructorCall() {
       this.getAConstructor().getACallToThisFunction() = result and
       not result.getLocation().getFile().toString().matches("%/boost/asio/%") and