(#11291) Update description of show_diff setting to match behavior as of 2.7.8

Issue #2744 changed the behavior of diff printing so that diffs pass through
Puppet's reporting/logging system, which changes both the situations where it
is useful to print diffs, and the security considerations of printing diffs. This
commit updates the setting's description accordingly.
(cherry picked from commit 901a6b26c06dbfd8fa52be9c5a7f24bcaae4144a)