Add "Probable Deviations From Markdown" Section To User Guide

[MartinPacker]

Now we have EPS support it seems reasonable to me to add a section warning users that certain features might not be compatible with other Markdown processors. There are very few of these but EPS support is one of them.

[MartinPacker]

102 Allow omitting trailing pipe in table lines would be another.

As would using dash counting to set table column widths.

[MartinPacker]

Also Taskpaper support.

[MartinPacker]

Done - for now.