LU-9727 doc: update llog_reader man page for Changelogs

LU-9727 doc: update llog_reader man page for Changelogs

Add new paragraph in llog_reader's man page to explain how to read
Changelogs with llog_reader, and add an example.

Test-Parameters: trivial
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I3e1123b9a5ac88334a370fd69c1d9d63597e16f7
Reviewed-on: https://review.whamcloud.com/30970
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>