gmessages: Clarify documentation about aborting on fatal log levels

Clarify that we actually raise SIGTRAP rather than calling abort(). We
haven’t called abort() since about 2011, when commit
a04efe6a changed the logic to use
SIGTRAP to make it possible to skip past fatal log messages in the
debugger if they weren’t relevant to the problem being debugged.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

GNOME/glib#1448
9 jobs for 1448-g-error-secretly-doesnt-abort in 20 minutes and 18 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #88502
cross-android_api21_arm64

00:01:37

passed #88503
cross-android_api28_arm64

00:01:38

passed #88504
cross-mingw64

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passed #88538
fedora-x86_64

00:04:21

manual #88507
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #88505
win32
msys2-mingw32

00:08:09

passed #88506
win32
vs2017-x64

00:05:48

failed #88501
fedora-x86_64

00:03:56

 
  Coverage
passed #88508
coverage

00:01:37

73.3%