bsdinstall: Provide help text for partitioning options

Includes commentary of when ZFS works well by default (>= 8GB RAM),
and where to go for information on ZFS tuning if required.

Also hoist the options text to the top of script as variables
(will help with future international translations).

Reviewed by: philip, dteske, karels, imp, emaste
Approved by: rgrimes
Differential Revision: https://reviews.freebsd.org/D23224