DOC: Flexible loads feasible domains

[benoit9126]

Location of the document
Flexible load section

Documentation problem
It seems that several elements are not clear in the documentation:

• Add more explanations to the feasible domains section.
• Maybe add some plot with some (P, Q) points per voltage in a wide range of voltages to see them in the (P,Q) disc. (for another issue: maybe allow the user to plot such figures by adding a plot method to FlexibleLoad class)
  
• Emphasize the fact that the projection may reduce P or Q or both values. Maybe add some control curves (P(V) or Q(V)) with the effect of the projection. Currently, there are only theoretical curves.
• Change the word circle in favour of disc. When the (P, Q) point lies in the disc, no projection is performed.
• Emphasize that s_max defines the maximum possible consumed or produced reactive power. Later, we plan to allow the user to define his own values. See ENH: Maximum produced/consumed reactive power #126

@YassineAbdelouadoud