doc: xfail doesn't mean testing a failure condition.

* doc/automake.texi (Generalities about Testing): mention that
xfail is not about testing a failure condition. See thread around:
https://lists.gnu.org/archive/html/automake/2025-07/msg00008.html

diff --git a/doc/automake.texi b/doc/automake.texi
index 83599a72208ad08508e74996dd7e3429ea77d7e5..27b5203490af716ef3c10c5569132f3e765e85cd 100644 (file)
--- a/doc/automake.texi
+++ b/doc/automake.texi
@@ -9600,14 +9600,21 @@ all of the testsuite has been run.
 @cindex xpass
 @cindex unexpected pass
 @cindex unexpected test pass
-It's not uncommon, especially during early development stages, that some
-tests fail for known reasons, and that the developer doesn't want to
-tackle these failures immediately (this is especially true when the
+It's not uncommon, especially during early development stages, that
+some tests fail for known reasons, and that the developer doesn't want
+to tackle these failures immediately (this is especially true when the
 failing tests deal with corner cases).  In this situation, the better
 policy is to declare that each of those failures is an @emph{expected
-failure} (or @emph{xfail}).  In case a test that is expected to fail ends
-up passing instead, many testing environments will flag the result as a
-special kind of failure called @emph{unexpected pass} (or @emph{xpass}).
+failure} (or @emph{xfail}).  When writing such a test, it is highly
+desirable to note in the test what has to happen to make the test
+succeed.  In case a test that is expected to fail ends up passing
+instead, many testing environments will flag the result as a special
+kind of failure called @emph{unexpected pass} (or @emph{xpass}).
+
+An expected failure result is not related to the contents of the test
+itself.  For example, a test might check whether a given system call
+correctly returns failure given an invalid argument.  If so, then the
+@emph{test} passes, even though it was testing a failure condition.
 
 @cindex hard error
 @cindex Distinction between errors and failures in testsuites