videometa: document how the meta can be used to negotiate buffers layout

I'm going to use this new API in gst-omx so an encoder can requests
v4l2src to produce buffers matching the encoder stride and slice heights
preventing copies of incoming buffers.
21 jobs for alig-negot-master in 36 minutes and 57 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #464046
gst indent

00:00:33

passed #464045
manifest

00:00:37

 
  Build
passed #464052
build cerbero android universal

00:09:21

passed #464053
build cerbero cross win32

00:08:04

passed #464054
build cerbero cross win64

00:05:03

passed #464051
build cerbero fedora x86_64

00:05:49

manual #464056
gst-ios-12.2 allowed to fail manual
build cerbero ios universal
passed #464055
gst-macos-10.14
build cerbero macos x86_64

00:06:51

passed #464047
build fedora x86_64

00:10:13

passed #464048
build nodebug fedora x86_64

00:05:27

passed #464049
build static fedora x86_64

00:12:50

passed #464050
build static nodebug fedora x86_64

00:11:56

 
  Test
failed #464057
check fedora

00:10:27

passed #464058
integration testsuites fedora

00:17:24

passed #464062
valgrind bad

00:06:19

passed #464059
valgrind base

00:13:04

passed #464063
valgrind ges

00:04:39

passed #464060
valgrind good

00:23:28

passed #464061
valgrind ugly

00:02:41

 
  Integrate
skipped #464064
android universal examples
skipped #464065
documentation
 
Name Stage Failure
failed
check fedora Test
           Total: 3112
Uploading artifacts...
gst-build/build/meson-logs/: found 2 matching files
validate-logs: found 3530 matching files

Uploading artifacts to coordinator... ok
id=464057 responseStatus=201 Created token=HA2NeJfK
Uploading artifacts...
validate-logs/*.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=464057 responseStatus=201 Created token=HA2NeJfK
ERROR: Job failed: exit code 1