v4l2codecs: Populate the documentation cache

Like many codecs, v4l2codecs plugin produce element base on platform support. Though it target embedded system and there is no system to support them all, which makes updating the cache very complex. So instead, I introduce GST_V4L2_CODEC_GEN_DOC=1 environment. In that mode, non-functional element are expose (they fail cleanly for what its worth). This way, I have a method on my PC to update the cache whenever need to. Its in short a maintainer tool. The doc can be populated by:

# gst-env is a short for long devenv call, but gst-env.py can do too
GST_V4L2_CODEC_GEN_DOC=1 LANG=C gst-env ninja -C build/ plugins_doc_cache

And then I add the bit I care about and update. In fact, this environment is also set now by build-doc.sh, so that any diff can be reported.