[monodoc] Clarified message when a backing file isn't found

If the backing file doesn't exist, it will be ignored.
The previous message made it sound like this is a serious problem, which confused people as it is the last message that is
printed when building Mono (because of a reference to 'DoesNotExist' in docs/toc.xml) while in reality everything is fine.
Merge pull request #1153 from akoeplinger/fix-doc-message

[monodoc] Clarified message when a backing file isn't found