pg_upgrade documentation improvement patch

I was upgrading a cluster, went on the IRC channel to clarify a few things,
the following discussion ensued:

yuri_niyazov: Hi. I am trying to upgrade to 9.5. The instructions for
pg_upgrade include the following bit:
[4:35pm] yuri_niyazov: "Again, use compatible initdb flags that match the
old cluster" - this is when running initdb before running pg_upgrade
[4:36pm] RhodiumToad: same encoding, locale, and checksum settings
[4:36pm] yuri_niyazov: Can I recover those from the data directory? The
previous initdb was ran 3 admins ago
[4:37pm] RhodiumToad: psql -l against the old server and look at the
encoding and locale for template0
[4:37pm] RhodiumToad: checksums are almost certainly not enabled, but you
can check that by running pg_controldata against the old data dir
[4:38pm] yuri_niyazov: thank you!
[4:38pm] RhodiumToad: (if the old server is too old for checksums, then
don't enable them in the new initdb)

I took RhodiumToad's comments and made them into a patch to pg_upgrade's
documentation. Patch was created via "git diff --patience HEAD^ HEAD |
filterdiff --format=context" as recommended on
https://wiki.postgresql.org/wiki/Working_with_Git