Mention the assignment form in "Copyright Assignment"

* doc/emacs/trouble.texi (Copyright Assignment): Mention the
copyright assignment form explicitly.  Suggested by Konstantin
Kharlamov <hi-angel@yandex.ru>.
1 job for emacs-26 in 37 minutes and 50 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #1381
test

00:37:50