Add info for TPLs to testsuite doc.

doc/developers/testsuite.html

@@ -449,6 +449,11 @@ <h3>Restricting tests to build configurations</h3>
 category/test.[with_&lt;string&gt;(&lt;=|&gt;=|=|&lt;|&gt;)&lt;on|off|version&gt;.]*
               [mpirun=&lt;N|all&gt;.][threads=&lt;N|all&gt;.][expect=&lt;y&gt;.][exclusive.][&lt;debug|release&gt;.](output|run_only)
 </pre>
+      Note that for CMake variables switching subpackages of third party libraries
+      like <code>DEAL_II_TRILINOS_WITH_BELOS</code> or <code>DEAL_II_PETSC_WITH_HYPRE</code>
+      the correct syntax is adding an additional <code>with_</code>,
+      i.e. <code>with_trilinos_with_belos=on|off</code>.</br></br>
+
       Normally, a test will be set up so that it runs twice, once in debug and
       once in release configuration.
       If a specific test can only be run in debug or release configurations but