Minor improvements to zpoolconcepts.7

• • Fixed one typo (effects -> affects)
  • Re-worded raidz description to make it clearer that it is not quite the same as RAID5, though similar
  • Clarified that data is not necessarily written in a static stripe width
  • Minor grammar consistency improvement
  • Noted that "volumes" means zvols
  • Fixed a couple of split infinitives
  • Clarified that hot spares come from the same pool they were assigned to
• "we" -> ZFS
• Fixed warnings thrown by mandoc, and removed unnecessary wordiness in one fixed line.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brandon Thetford <brandon@dodecatec.com>
Closes #14726