Improve HTMLScriptElement::didMoveToNewDocument() comment.

      Clarify when contextDocument() will return no Document; comment-only
change.

R=haraken
BUG=443115
NOTRY=true

Review URL: https://codereview.chromium.org/827233002

git-svn-id: svn://svn.chromium.org/blink/trunk@187777 bbb929c8-8fbe-4397-9dbb-9b2b20218538