grefstring: Document GRefString type

This symbol has been added to glib-sections.txt in 93f3202a 
but lacks a gtk-doc comment block.

So, let's add it.
9 jobs from wip/antoniof/document_grefstring_type in 8 minutes and 30 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #165517
cross-android_api21_arm64

00:01:35

passed #165518
cross-android_api28_arm64

00:00:59

passed #165519
cross-mingw64

00:01:23

failed #165516
fedora-autotools-x86_64

00:04:22

passed #165515
fedora-x86_64

00:05:33

failed #165522
freebsd-11
freebsd-11-x86_64

00:04:30

passed #165520
win32
msys2-mingw32

00:08:27

passed #165521
win32
vs2017-x64

00:07:57

 
  Coverage
skipped #165523
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 233 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=165516 responseStatus=201 Created token=3__Z1zGV
ERROR: Job failed: exit code 1
failed
freebsd-11-x86_64 Build
TIMEOUT:    0

Full log written to /var/tmp/gitlab_runner/builds/3fe11159/0/GNOME/glib/_build/meson-logs/testlog.txt
Uploading artifacts...
_build/config.h: found 1 matching files
_build/glib/glibconfig.h: found 1 matching files
_build/meson-logs: found 4 matching files
Uploading artifacts to coordinator... ok id=165522 responseStatus=201 Created token=x7pYiQqC
ERROR: Job failed: exit status 1