hugolib: Fix regression of .Truncated evaluation in manual summaries

hugolib: Fix regression of .Truncated evaluation in manual summaries

This fixes the behavior of .Truncated that was introduced with commit
bef496b97e1dc2df8c52b080152d539a58752801 which was later broken.  The
desired behavior is that .Truncated would evaluate to false when there
was nothing after the user defined summary marker.

This also adds a simple unit test to ensure that this feature isn't
broken again.  The check for content after the user defined summary
marker is done on the raw content instead of the working copy because
some of the markup renderers add elements after the marker, making it
difficult to determine if there is actually any content.

The behavior (evaluating to false when there is no content, just
summary) is also now documented.