[FEATURE] Documentation for double logistic selectivity

[JaneSullivan-NOAA]

See original issue here

[JaneSullivan-NOAA]

\f$` \frac{1.0}{ 1.0 + exp(-1.0 * slope_{asc} (x - median_{asc}))} \left(1-\frac{1.0}{ 1.0 + exp(-1.0 * slope_{desc} (x - median_{desc}))}  \right)\f$

[k-doering-NOAA]

Yay for LaTex math equations! Github will even format them nicely on issues:

$\frac{1.0}{ 1.0 + exp(-1.0 slope{asc} (x - median{asc}))} \left(1-\frac{1.0}{ 1.0 + exp(-1.0 slope{desc} (x - median{desc}))} \right)$

Note this is also possible (not sure if it is more readable, after previewing):

$\frac{1.0}{1.0 + e^{-1.0 slope{asc} (x - median{asc})}} \left(1-\frac{1.0}{ 1.0 + e^{-1.0 slope{desc} (x - median{desc})}} \right)$

Latex version:

$\frac{1.0}{1.0 + e^{-1.0 * slope_{asc} (x - median_{asc})}} \left(1-\frac{1.0}{ 1.0 + e^{-1.0 * slope_{desc} (x - median_{desc})}}  \right)$