pipeline: clarify that applications should handle bus messages

The pipeline posts messages on the bus even if an application does not
handle the messages. This is expected behavior but may leak messages if
the messages are not handled.

Clarify the documentation.
35 jobs for !680 with clarify-pipeline-bus-documentation in 2 minutes and 31 seconds (queued for 3 seconds)
detached
Status Job ID Name Coverage
  Build Docker
passed #5163301
alpine amd64 manifest builder docker

00:00:27

passed #5163300
android docker

00:01:42

passed #5163299
cerbero fedora amd64 docker

00:01:56

passed #5163298
fedora amd64 docker

00:02:23

passed #5163302
gst-indent amd64 docker

00:00:27

 
  Preparation
passed #5163304
gst indent

00:00:07

manual #5163303
manual
manifest
 
  Build
created #5163313
gstreamer
build cerbero cross-android universal
created #5163317
gst-ios-13.2
build cerbero cross-ios universal
created #5163314
gstreamer
build cerbero cross win32
created #5163315
gstreamer
build cerbero cross win64
created #5163312
gstreamer
build cerbero fedora x86_64
created #5163316
gst-macos-10.15
build cerbero macos x86_64
created #5163318
1809 gstreamer-windows windows docker
build cerbero msvc x86_64
created #5163308
build clang fedora x86_64
created #5163311
1809 windows docker allowed to fail manual
build msys2
created #5163305
build nodebug fedora x86_64
created #5163306
build static fedora x86_64
created #5163307
build static nodebug fedora x86_64
created #5163309
1809 windows docker
build vs2017 amd64
created #5163310
1809 windows docker
build vs2017 x86
 
  Test
created #5163319
gstreamer
check fedora
created #5163320
gstreamer
integration testsuites fedora 1/4
created #5163321
gstreamer
integration testsuites fedora 2/4
created #5163322
gstreamer
integration testsuites fedora 3/4
created #5163323
gstreamer
integration testsuites fedora 4/4
created #5163328
gstreamer
valgrind bad
created #5163325
gstreamer
valgrind base
created #5163324
gstreamer
valgrind core
created #5163329
gstreamer
valgrind ges
created #5163326
gstreamer
valgrind good
created #5163327
gstreamer
valgrind ugly
 
  Integrate
created #5163330
cross-android universal examples
created #5163332
gst-ios-13.2
cross-ios universal examples
created #5163331
documentation