Extract line annotation from filter qualifiers in a type-friendly way

A recently submitted patch (78ce891).used function `get_anno/1` to extract
the line annotation from filter qualifiers in comprehensions, but this did not
respect the spec of this function and resulted in a dialyzer warning.
To make the code more type-friendly, an appropriate `get_qual_line/1`
function was introduced.  The new function also ensures that the
qualifiers are of the appropriate form.

While at it, also simplified an Edoc comment.