Extend docs

l3build.dtx

@@ -437,6 +437,7 @@
 % To perform the check, the engine typesets each test up to \var{checkruns} times.
 % More detail on this in the documentation on |save|.
 % Options passed to the binary are defined in the variable \var{checkopts}.
+% These are applied in addition to any 
 %
 % By default, |texmf| trees are searched for input files when checking. This can be disabled by setting \var{checksearch} to |false|: isolation provides confidence that the tests cannot accidentally be running with incorrect files installed in the main distribution or |hometexmf|.
 %