Add admonition about info-file w/r/t paired end mode

[odoublewen]

I found issue https://github.com/marcelm/cutadapt/issues/302 that makes clear that --info-file does not currently fully support paired-end mode.

I suggest adding a little admonition to the docs to make this clear. (Apologies if this information already exists somewhere in the docs -- I did a quick search but I couldn't find any mentions, aside from the above issue.)

[marcelm]

Thanks, not sure whether this is mentioned anywhere else, but it’s a good idea to point it out even in that section!