These commands work similarly to the 'flush-lines' command, but add the
lines to the kill ring as a single item.

For example, when working with text lists, such as in

     - Documentation: Do thing 1
     - Source: Add new function
     - Documentation: Do thing 2

if one wanted to copy only the lines containing "Documentation", while
leaving the buffer untouched, the 'copy-lines' command could be used.