[doc] extend subopt discussion

[martin-raden]

tackle user feedback:

The only aspect I think confused me was whether sub-optimal matches were talking only about the same position in the UTR/miRNA - with differences in binding within this, or whether you would report just the top hit for each mRNA sequence, unless sub-optimal positions were enabled. Ie. would I get back more than one hit per mRNA in different positions along the mRNA or not