Update documentations of ACTION_PICK_IMAGES and EXTRA_MEDIA_CAPABILITIES

Update documentations of MediaStore to mention that EXTRA_MEDIA_CAPABILITIES
can be used with ACTION_PICK_IMAGES to trigger transcoding.

For details see: go/photopicker-hdr-video-transcoding.

Test: manual
Bug: 365461942
Flag: com.android.providers.media.flags.enable_photopicker_transcoding
Change-Id: I53e69c610d8776021a3ecf20b9cfd9629aaf0ff3