padtemplate: expose getters and setters "documentation caps"

This can be used in elements where the caps of pad templates
are dynamically generated and dependent on the environment.

An example is x265enc.
30 jobs for master in 16 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #2978830
gst indent

00:00:40

passed #2978829
manual
manifest

00:00:24

 
  Build
passed #2978839
gstreamer
build cerbero cross-android universal

00:10:30

canceled #2978843
gst-ios-13.2
build cerbero cross-ios universal

00:14:40

passed #2978840
gstreamer
build cerbero cross win32

00:06:36

passed #2978841
gstreamer
build cerbero cross win64

00:06:55

passed #2978838
gstreamer
build cerbero fedora x86_64

00:04:59

passed #2978842
gst-macos-10.15
build cerbero macos x86_64

00:10:58

passed #2978834
build clang fedora x86_64

00:05:37

manual #2978837
1809 windows docker allowed to fail manual
build msys2
passed #2978831
build nodebug fedora x86_64

00:03:52

passed #2978832
build static fedora x86_64

00:03:51

passed #2978833
build static nodebug fedora x86_64

00:03:20

passed #2978835
1809 windows docker
build vs2017 amd64

00:12:15

passed #2978836
1809 windows docker
build vs2017 x86

00:12:15

 
  Test
canceled #2978844
gstreamer
check fedora

00:14:44

canceled #2978845
gstreamer
integration testsuites fedora 1/4

00:14:43

canceled #2978846
gstreamer
integration testsuites fedora 2/4

00:14:43

canceled #2978847
gstreamer
integration testsuites fedora 3/4

00:13:00

canceled #2978848
gstreamer
integration testsuites fedora 4/4

00:11:27

canceled #2978853
gstreamer
valgrind bad

00:04:11

canceled #2978850
gstreamer
valgrind base

00:08:06

canceled #2978849
gstreamer
valgrind core

00:09:44

canceled #2978854
gstreamer
valgrind ges

00:03:38

canceled #2978851
gstreamer
valgrind good

00:07:46

canceled #2978852
gstreamer
valgrind ugly

00:04:38

 
  Integrate
canceled #2978855
cross-android universal examples

00:04:13

canceled #2978857
gst-ios-13.2
cross-ios universal examples

canceled #2979486
documentation

00:00:12

failed #2978856
documentation

00:07:00