truncate.1: Polish title and examples

+ describe better
+ switch examples to human readable sizes, with consistent spacing
+ Downsize ... in 5 Megabytes >> Downsize ... by 5 Megabytes
+ remove prompt from lone example with prompt for consistency
+ remove x permissions from kernel example
+ examples now fit on standard console without wrapping

While here:
+ fold a line to eliminate linter warning + tag spdx
+ add -nosplit to AUTHORS to eliminate a rendering glitch

MFC after: 3 days
Discussed with: asomers, jhb, maxim
Reviewed by: imp (previous version)
Closes: https://github.com/freebsd/freebsd-src/pull/1568

(cherry picked from commit 91ad27bcfc496393ba88f39ca13cdf1c59a7f487)