makechrootpkg: Fix function usage comments

A couple of the comments noting which globals are used by functions are
outdated/wrong.

 - download_sources() : Remove USER from the list.  It was always wrong.
   Originally, it should have been SUDO_USER (not USER), but I should have
   removed it entirely in 4f23609.

 - move_products() : Add SRCPKGDEST to the list.  Though the commit adding
   the comment was only recently upstreamed (as 2fd5931), it originated in
   2013 in a commit that has since  been rebased many times.  Anyway, in
   this rebasing, it missed move_products() starting to pay attention to
   SRCPKGDEST in fd1be1b (since nothing made git think there was a
   "conflict").

makechrootpkg.in

@@ -245,7 +245,6 @@ _chrootnamcap() {
 # Usage: download_sources $copydir $makepkg_user
 # Globals:
 #  - SRCDEST
-#  - USER
 download_sources() {
 	local copydir=$1
 	local makepkg_user=$2
@@ -267,6 +266,7 @@ download_sources() {
 # Globals:
 #  - PKGDEST
 #  - LOGDEST
+#  - SRCPKGDEST
 move_products() {
 	local copydir=$1
 	local src_owner=$2