Close PR docs/40851

[Niclas Zeising]

I was looking through some old PRs in the gnats database and tripped 
over PR docs/40851 http://www.freebsd.org/cgi/query-pr.cgi?pr=docs/40851 
. It mentions the addition of mergemaster -p to the instructions on how 
to upgrade a system in UPDATING. Mergemaster -p is already mentioned in 
UPDATING in RELENG_5, RELENG_6 and HEAD. It is still lacking in in 
UPDATING in RELENG_4 but I think an addition is unnecessary since 
RELENG_4 is close to EOL. Therefore, I believe we can close this PR.

Regards!
//Niclas