More duration time clarification text

Add a warning to the subtitle about the data being unreliable.

Closer investigation of strange loading time behavior shows
that mediaWikiLoadComplete fires much sooner than domComplete
(and significantly sooner than even domInteractive); it is not clear
which one is a good approximation of the image becoming visible.

Change-Id: Ie67afc8e1f3c215d88e6b27faea79b8f65b39e97