Document FEATURES=userpatches in make.conf(5)
Adapted from comment in eapply_user() in phase-helpers.sh
Signed-off-by: Robin Couto <dbtx@ozono.us>

--- a/man/make.conf.5
+++ b/man/make.conf.5
@@ -714,6 +714,31 @@
 When portage is run as root, drop privileges to portage:portage during the
 fetching of package sources.
 .TP
+.B userpatches
+Apply patches in /etc/portage/patches/ during the prepare phase. The inner
+directory structure follows atom syntax in ebuild(5). Filenames ending
+in .diff and .patch found in all matching subdirectories are combined into a
+list of the patch basenames. Those in more-specific subdirectories override
+patches of the same basename found in less-specific subdirectories. An empty
+patch (or /dev/null symlink) negates a patch with the same basename found in
+a less-specific subdirectory. In order, from most to least specific, these
+subdirectories will be matched to the current ebuild:
+
+	${CATEGORY}/${P}-${PR}:${SLOT}
+.br
+	${CATEGORY}/${P}-${PR}
+.br
+	${CATEGORY}/${P}:${SLOT}
+.br
+	${CATEGORY}/${P}
+.br
+	${CATEGORY}/${PN}:${SLOT}
+.br
+	${CATEGORY}/${PN}
+
+For the first two forms, ${P}-r0 also matches and overrides ${P}, if the ebuild
+has no revision number. This feature is enabled by default.
+.TP
 .B userpriv
 Allow portage to drop root privileges and compile packages as
 portage:portage without a sandbox (unless \fIusersandbox\fR is also used).