freebsd-update: Clarify help text

Improve help text to include example release numbers for reference
and clarify the -F option.

PR:		231185, 214619
Submitted by:	Gerald Aryeetey <aryeeteygerald_rogers.com>
Reviewed by:	delphij, rgrimes
MFC after:	1 month
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18944

usr.sbin/freebsd-update/freebsd-update.8

@@ -89,13 +89,13 @@ Read configuration options from
 .It Fl F
 Force
 .Nm Cm fetch
-to proceed where it normally would not, such as an unfinished upgrade
+to proceed in the case of an unfinished upgrade.
 .It Fl k Ar KEY
 Trust an RSA key with SHA256 of
 .Ar KEY .
 (default: read value from configuration file.)
 .It Fl r Ar newrelease
-Specify the new release to which
+Specify the new release (e.g. 11.2-RELEASE) to which
 .Nm
 should upgrade (upgrade command only).
 .It Fl s Ar server

usr.sbin/freebsd-update/freebsd-update.sh

@@ -45,7 +45,8 @@ Options:
                   (default: /var/db/freebsd-update/)
   -f conffile  -- Read configuration options from conffile
                   (default: /etc/freebsd-update.conf)
-  -F           -- Force a fetch operation to proceed
+  -F           -- Force a fetch operation to proceed in the
+                  case of an unfinished upgrade
   -k KEY       -- Trust an RSA key with SHA256 hash of KEY
   -r release   -- Target for upgrade (e.g., 11.1-RELEASE)
   -s server    -- Server from which to fetch updates