geom manuals: Clarify units

The gpart manual says that sizes are specified in blocks, unless an SI
unit suffix is provided. This confuses new operators because GEOM uses
binary bytes, a large difference at modern storage pool sizes. Rewrite
suffixes in all GEOM manuals to consistently clarify this, matching what
we and the rest of the industry have been doing in other documentation.
While here, use non-breaking spaces between numbers and units, unless
they are already written with a hyphen.

MFC after: 3 days
Reviewed by: fuz
Reported by: bbaovanc <bbaovanc@bbaovanc.com>
Differential Revision: https://reviews.freebsd.org/D56534