doc: change topic phrase 'disappear' to 'fade out'

Change all instances which describe topics as "disappearing" to the more
accurate "fade out" as the information is still available, but no longer
used.

--HG--
branch : stable
8 jobs for topic/stable/ctuffli-doc-from-mq in 6 minutes and 10 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed checks-py2 #171004

00:01:28

passed checks-py3 #171005

00:01:20

passed doc #171010

00:00:30

manual pytype #171003
allowed to fail manual
passed tests-py2-cext #171006

00:04:04

passed tests-py2-pure #171007

00:03:28

passed tests-py3-cext #171008

00:05:36

passed tests-py3-pure #171009

00:06:08