DOC: Document known differences between synphot and PySynphot

[pllim]

As discussed with @deustua and @tddesjardins , it would be good to write down known differences between this package and PySynphot. Perhaps as a new column in the conversion guide table. Examples:

• synphot does not allow spectrum objects with magnitude flux units but PySynphot does.
• synphot is not affected by <insert bug>.
• synphot calculates <insert property> with <this> but PySynphot used <that>.

This would probably need group effort and be done in pieces over time. Maybe we can fit this into the INS "hack days"?

[pllim]

Maybe this is superseded by Develop Notebooks with Synphot Functionality?