revset: fix example describing how ordering is determined
It was 'X & !Y' before.
--- a/mercurial/revset.py Wed Aug 30 22:32:47 2017 +0900
+++ b/mercurial/revset.py Wed Aug 30 23:53:30 2017 +0900
@@ -64,13 +64,13 @@
#
# 'any' means the order doesn't matter. For instance,
#
-# (X & Y) | ancestors(Z)
-# ^ ^
-# any any
+# (X & !Y) | ancestors(Z)
+# ^ ^
+# any any
#
-# For 'X & Y', 'X' decides order so the order of 'Y' does not matter. For
-# 'ancestors(Z)', Z's order does not matter since 'ancestors' does not care
-# about the order of its argument.
+# For 'X & !Y', 'X' decides the order and 'Y' is subtracted from 'X', so the
+# order of 'Y' does not matter. For 'ancestors(Z)', Z's order does not matter
+# since 'ancestors' does not care about the order of its argument.
#
# Currently, most revsets do not care about the order, so 'define' is
# equivalent to 'follow' for them, and the resulting order is based on the