Document PORTAGE_REPOSITORIES.
diff --git a/man/portage.5 b/man/portage.5
index ad801ba..0aac569 100644
--- a/man/portage.5
+++ b/man/portage.5
@@ -765,6 +765,9 @@
 .TP
 .BR repos.conf
 Specifies \fIsite\-specific\fR repository configuration information.
+.br
+Configuration specified in \fBrepos.conf\fR can be overriden by \fBPORTAGE_REPOSITORIES\fR
+environmental variable, which has the same format as \fBrepos.conf\fR.
 
 .I Format:
 .nf