libgimp: add missing documentation for gimp_procedure_dialog_new().

This is important for introspection in particular with annotations.
16 jobs for master in 66 minutes and 38 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #1122912
build-image

00:26:01

passed #1122914
build-image-win32

00:17:32

passed #1122913
build-image-win64

00:17:04

 
  Dependencies
passed #1122915
deps-debian

00:05:01

passed #1122917
deps-win32

00:20:09

passed #1122916
deps-win64

00:19:56

 
  Gimp
passed #1122918
gimp-autotools-debian

00:21:59

passed #1122921
gimp-clang-debian

00:17:41

passed #1122919
gimp-distcheck-debian

00:31:53

passed #1122920
gimp-meson-debian

00:13:43

passed #1122923
gimp-win32

00:09:04

passed #1122922
gimp-win64

00:20:05

 
  Analysis
passed #1122924
cppcheck

00:01:48

 
  Distribution
passed #1122925
sources

00:02:05

passed #1122927
win32-nightly

00:15:11

passed #1122926
win64-nightly

00:09:28