binding: Clarify the use of g_object_unref() to remove a binding

Conceptually the binding is kept alive as long as both the source and
target exist. This means that an API user needs to take some care to
either hold a reference or only use a pointer to the binding as long as
also holding references to both objects.

Clarify the documentation a bit.
8 jobs from doc-binding-improvement in 13 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #162170
cross-android_api21_arm64

00:03:37

passed #162171
cross-android_api28_arm64

00:04:24

passed #162172
cross-mingw64

00:06:24

failed #162169
fedora-autotools-x86_64

00:07:24

passed #162168
fedora-x86_64

00:06:03

passed #162173
win32
msys2-mingw32

00:12:12

passed #162174
win32
vs2017-x64

00:07:59

 
  Coverage
skipped #162175
coverage
 
Name Stage Failure
failed
fedora-autotools-x86_64 Build
_build/glib/glibconfig.h: found 1 matching files   
_build/*.log: found 3 matching files
_build/*/*.log: found 6 matching files
_build/*/*/*.log: found 209 matching files
_build/*/*/*/*.log: found 1 matching files
WARNING: _build/*/*/*/*/*.log: no matching files
WARNING: _build/*/*/*/*/*/*.log: no matching files
Uploading artifacts to coordinator... ok id=162169 responseStatus=201 Created token=HS-NmED_
ERROR: Job failed: exit code 1