C++: Document external package tables in dbscheme

cpp/ql/src/semmlecode.cpp.dbscheme

@@ -181,17 +181,24 @@ tokens(
     int endColumn : int ref
 );
 
-/*
- * Mapping of header files to packages
+/**
+ * Information about packages that provide code used during compilation.
+ * The `id` is just a unique identifier.
+ * The `namespace` is typically the name of the package manager that
+ * provided the package (e.g. "dpkg" or "yum").
+ * The `package_name` is the name of the package, and `version` is its
+ * version (as a string).
  */
-
 external_packages(
     unique int id: @external_package,
-    string namespace : string ref, // "dpkg", "yum", ...
+    string namespace : string ref,
     string package_name : string ref,
     string version : string ref
 );
 
+/**
+ * Holds if File `fileid` was provided by package `package`.
+ */
 header_to_external_package(
     int fileid : @file ref,
     int package : @external_package ref