virtualx.eclass: Add verbose warning about nonfatal call

Gentoo-Bug: https://bugs.gentoo.org/show_bug.cgi?id=517976

Signed-off-by: Justin Lecher <jlec@gentoo.org>

eclass/virtualx.eclass

@@ -111,6 +111,16 @@ virtualmake() {
 # @DESCRIPTION:
 # Start new Xvfb session and run commands in it.
 #
+# IMPORTANT: The command is run nonfatal !!!
+#
+# This means we are checking for the return code and raise an exception if it
+# isn't 0. So you need to make sure that all commands return a proper
+# code and not just die. All eclass function used should support nonfatal
+# calls properly.
+#
+# The rational behind this is the tear down of the started Xfvb session. A
+# straight die would leave a running session behind.
+#
 # Example:
 #
 # @CODE
@@ -124,6 +134,17 @@ virtualmake() {
 # 	virtx py.test --verbose
 # }
 # @CODE
+#
+# @CODE
+# my_test() {
+#   some_command
+#   return $?
+# }
+#
+# src_test() {
+#   virtx my_test
+# }
+# @CODE
 virtx() {
 	debug-print-function ${FUNCNAME} "$@"