docs: fix a misunderstanding in g_type_add_interface_*

The previous documentation said this:
  g_type_add_interface_static:
    "Adds the static interface_type to instantiable_type"
  g_type_add_interface_dynamic:
    "Adds the dynamic interface_type to instantiable_type"

The above suggests that if one is adding a static interface to a dynamic
object, one should use g_type_add_interface_static because the interface
is static, but the code and usage (with the newly added
G_IMPLEMENTS_INTERFACE_DYNAMIC) imply that this is wrong, and that
what matters is whether the *instanciable_type* is dynamic or not.

Hence this patch moves the "static" and "dynamic" words close to
"instantiable_type".

Closes issue #259
11 jobs for fix_a_misunderstanding_in_gtypeaddinterface in 37 minutes and 21 seconds (queued for 5 minutes and 23 seconds)
Status Job ID Name Coverage
  Build
passed #418204
cross-android_api21_arm64

00:02:24

passed #418205
cross-android_api28_arm64

00:03:28

passed #418206
cross-mingw64

00:04:21

passed #418202
debian-stable-x86_64

00:05:32

passed #418201
fedora-x86_64

00:07:41

passed #418203
G_DISABLE_ASSERT

00:05:05

passed #418207
win32
msys2-mingw32

00:09:19

passed #418208
win32
vs2017-x64

00:09:02

 
  Coverage
passed #418209
coverage

00:01:40

77.9%
 
  Analysis
passed #418211
scan-build

00:07:53

failed #418210
allowed to fail
valgrind

00:13:25

 
Name Stage Failure
failed
valgrind Analysis
Uploading artifacts...
_build/config.h: found 1 matching files
_build/glib/glibconfig.h: found 1 matching files

_build/meson-logs: found 7 matching files

Uploading artifacts to coordinator... ok
id=418210 responseStatus=201 Created token=NdyVgbxz
Uploading artifacts...
_build/valgrind-report.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=418210 responseStatus=201 Created token=NdyVgbxz
ERROR: Job failed: exit code 1