pad: Document that pad unlink function is called with pad lock held

Fixes #353
8 jobs for unlink-func-doc-update in 28 minutes and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Preparation
passed #113550
gst indent

00:00:36

passed #113549
manifest

00:00:39

 
  Build
passed #113551
build fedora x86_64

00:03:23

 
  Test
passed #113552
check fedora

00:04:13

 
  Full Builds
passed #113553
build android arm64 api28

00:03:10

passed #113555
build cerbero cross win32

00:10:00

passed #113556
build cerbero cross win64

00:08:35

passed #113554
build cerbero fedora x86_64

00:08:12