lib/csv.rb: Small documentation fix

This clarifies the documentation for the skip_blanks option to the CSV library.

The original documentation was confusing, since  "CSV will skip over any rows with no content" doesn't distinguish between "rows that are literally blank" and "rows that contain only column separators but no data." This revised documentation explains that the skip_blanks option only handles the former case, and gives suggestions about how to handle the latter.