Update instructions for manually creating a jail

Remove unneeded subshell
Remove csh examples, and explicitly use sh everywhere
Remove the GAMES set which on longer exists
Remove the DOCS set that may be removed in the future, it is not needed inside a jail

PR: 194993
Differential Revision: https://reviews.freebsd.org/D2116
Submitted by: alex@kaworu.ch (original)
Reviewed by: wblock
Approved by: bcr (mentor)
Sponsored by: ScaleEngine Inc.