lalsimulation conventions documentation improvements

The image https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/lalsiminspiral_orbitelements.svg has the x-axis pointing from body2 -> body1, which I think is correct.

https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimInspiral.h#L123 describes what seems to be the "old" (lalinference_o2) spin-conventions.
[x] Amended to be consistent with the image.

[x] https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimInspiral.h#L96: The text mean anomaly of periastron is meaningless (mean anomaly==0 at periastron, by defn)

[x] https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimInspiral.h#L65 -- fix typo 'eccentrocity'

[x] amend docu to state that Omega=longAscNode + pi/2, where 'longAscNode' is the value passed into ChooseTDWaveform.

[ ] https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimInspiral.h#L62 uses 'i', https://git.ligo.org/lscsoft/lalsuite/blob/master/lalsimulation/src/LALSimInspiral.h#L92 uses 'inclination'. Use the same parameter name in both cases. In both cases state that this angle is \iota in the @image.

[x] include explanation how phiRef is related to the angle Phi in @image.