Install docs into /usr/share/doc/wcd-${PVR}.

Bug: https://bugs.gentoo.org/802696

--- a/src/Makefile
+++ b/src/Makefile
@@ -61,7 +61,7 @@
 datarootdir     = $(prefix)/share
 datadir         = $(datarootdir)
 
-docsubdir       = $(PACKAGE)-$(VERSION)
+docsubdir       = $(PACKAGE)-$(PVR)
 docdir          = $(datarootdir)/doc/$(docsubdir)
 localedir       = $(datarootdir)/locale
 sysconfdir      = /etc