documentation: add section no strong and weak aliases

Author: ginsbach

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

@@ -123,4 +123,18 @@ You could give the predicate a more descriptive name as follows:
 
 .. code-block:: ql
 
-    predicate lessThanTen = isSmall/1;
+    predicate lessThanTen = isSmall/1;
+
+.. _weak_strong_aliases:
+
+Strong and weak aliases
+=======================
+
+Every alias is either **strong** or **weak**.
+An alias is **strong** if and only if it is a :ref:`type alias <type-aliases>` with :ref:`annotation <annotations>`
+``final``.
+During :ref:`name resolution <name-resolution>`, ambiguity between **weak** aliases of the same target is allowed,
+but ambiguity between distinct **strong** aliases or entities is invalid QL.
+For the purpose of applicative instantiation of :ref:`parameterised modules <parameterized-modules>` and
+`:ref:`parameterised module signatures <parameterized-module-signatures>`, **weak** aliases of instantiation
+arguments are considered equivalent, but distinct **strong** aliases result in separate instantiations.