Clarify that absolute URI, might be changed later.

Closes modelica#2701.

chapters/packages.tex

@@ -283,7 +283,7 @@ \subsection{The within Clause}\label{the-within-clause}
 \section{External resources}\label{external-resources}
 
 In order to reference external resources from documentation (such as links and images in html-text) and/or to reference images in the \lstinline!Bitmap! annotation (see \cref{bitmap}).
-URIs should be used, for example \filename{file:///} and the URI scheme \filename{modelica://} which can be used to retrieve resources associated with a package.
+Absolute URIs should be used, for example \filename{file:///} and the URI scheme \filename{modelica://} which can be used to retrieve resources associated with a package.
 According to the URI specification scheme names are case-insensitive, but the lower-case form should be used, that is \filename{Modelica://} is allowed but \filename{modelica://} is the recommended form.
 
 The Modelica-scheme\index{Modelica!URI scheme}\index{URI!Modelica} has the ability to reference a hierarchical structure of resources associated with packages.