update documentation in --help

urchin

@@ -384,17 +384,19 @@ particular test file once per shell.
 On each run,
 
 1. The TEST_SHELL environment variable is set to the particular shell.
-2. If the test file has a shebang line of "#!/bin/sh" or no shebang
-   line at all, the test script is also executed in that shell.
+2. If the test file lacks a shebang line, the test script is also
+   executed in that shell.
 
 The following flags affect how this multiple-shell testing is handled.
 
 -s, --shell <shell>      Tell Urchin to use a different list of shells.
                          (You can pass this flag multiple times.)
 
--n, --disable-cycling    Disable the cycling of shells; Urchin will execute
-                         test files directly, and it will not manipulate the
-                         TEST_SHELL environment variable.
+-n, --disable-cycling    Disable the cycling of shells; Urchin will
+                         execute test files ordinarily, implicitly using
+                         sh for files that lack shebang lines. It will
+                         set the TEST_SHELL variable to "/bin/sh" if and
+                         only if TEST_SHELL is empty or undefined.
 
 The following flags affect how Urchin processes tests.