document Instability in initial F near Fmsy

[k-doering-NOAA]

There is a non-monotonic relationship that can interfere with initial F estimation if the option to allow spawner-recruitment to operate inside initial year is used and if the input initial catch is near MSY. In the attached file there is a profile on initial F bracketing Fmsy. It shows, quite logically, that values of F greater or less than Fmsy give initial catch < MSY and that only init F = Fmsy produces an equilibrium catch equal to MSY. So there are two pathological problems:

if initial catch is near MSY (of course, SS has not yet calculated Fmsy yet), then the search for init F will encounter the non-monotonic situation
if initial catch is > MSY, then the mismatch between observed and estimated initial catch cannot be reduced by increasing init F further. However, it might be possible for SS to increase R0 or other parameters in order to increase MSY indirectly.

original issue nmfs-ost/ss3-source-code#18