Debian: Correctly set and use "docdir"

(cherry picked from commit 06920df4bb88260aba7d5bf1c2000d2e521bab3e)
Debian: Don't adjust path names that are correct by default

(cherry picked from commit f5944a17e44b1acd642aaeaf1ad551e3b0074fb5)
New "Debian release" 21.1-0ab2