Add documentation about differences in nodal-elements in pymapdl-mapdl

[germa89]

Because of some default configurations, there might be some differences between MAPDL and PyMAPDL nodal and/or elements results.

The topic to list are:

• Differences between MAPDL GUI and PyMAPDL
  • In listings - Assigned to @germa89 plus @mikerife support
  • In plots - Assigned to @pvargasm

See:

• Result Class pymapdl-reader#185
• Add info gathered by @pvargasm about nodal/element results