Clarify syntax for qualified names of operators in manual (#19237)

Clarify syntax for qualified names of operators

doc/manual/modules.rst

@@ -234,7 +234,13 @@ Namespace miscellanea
 ---------------------
 
 If a name is qualified (e.g. ``Base.sin``), then it can be accessed even if
-it is not exported. This is often useful when debugging.
+it is not exported. This is often useful when debugging. It can also have
+methods added to it by using the qualified name as the function name.
+However, due to syntactic ambiguities that arise, if you wish to add methods
+to a function in a different module whose name contains only symbols, such as
+an operator, ``Base.+`` for example, you must use ``Base.:+`` to refer to it.
+If the operator is more than one character in length you must surround it in
+brackets, such as: ``Base.:(==)``.
 
 Macro names are written with ``@`` in import and export statements, e.g.
 ``import Mod.@mac``. Macros in other modules can be invoked as ``Mod.@mac``