emerge: add reference to the portage(5) man page when failing

For example, the current licensing error message looks like:

 The following license changes (package.license) are necessary to proceed:
 #required by quake3-bin (argument)
 >=games-fps/quake3-bin-1.32c-r1 GPL-2 Q3AEULA

If you don't know much about licensing issues, this error message
doesn't help.  Instead, give references to the man page so people
can easily delve further.  Now it looks like:

 The following license changes are necessary to proceed:
  (see package.license in the portage(5) man page for more details)
 #required by quake3-bin (argument)
 >=games-fps/quake3-bin-1.32c-r1 GPL-2 Q3AEULA

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
portage(5): add more pointers to make.conf

Signed-off-by: Mike Frysinger <vapier@gentoo.org>