Add documentation for modicifcations:

- LoadModification
- PercentChangeLoadModification
- SetGeneratorToLocalRegulation

Signed-off-by: CARON Alice <alice.caron@rte-france.com>

docs/grid_features/network_modifications.md

@@ -261,7 +261,9 @@ The builder takes the ID of the substation as input.
 <span style="color: red">TODO</span>
 
 ### LoadModification
-<span style="color: red">TODO</span>
+This class is used to change the `P` and `Q` values of the load to new fixed values.  
+If the `relativeValue` is `true` then the `P0` and `Q0` given values are relative to the current `P` and `Q` values and will be applied as a sum:  Pnew = P0 + P / Qnew = Q0 + Q  
+If not then the `P0` and `Q0` values are given as absolute values and will replace the current `P` and `Q` values: Pnew = P0 / Qnew = Q0
 
 ### NetworkModificationList
 <span style="color: red">TODO</span>
@@ -270,7 +272,7 @@ The builder takes the ID of the substation as input.
 <span style="color: red">TODO</span>
 
 ### PercentChangeLoadModification
-<span style="color: red">TODO</span>
+This class is used to add or remove a percentage of the P and Q of the load. The percentage to add or remove for P and Q cannot be more than 100.
 
 ### PhaseShifterOptimizeTap
 <span style="color: red">TODO</span>
@@ -294,7 +296,7 @@ The builder takes the ID of the substation as input.
 <span style="color: red">TODO</span>
 
 ### SetGeneratorToLocalRegulation
-<span style="color: red">TODO</span>
+This class is used to set the generator regulating terminal to a local regulation. The target voltage value is set to the same value for all the generators of the bus that are regulating locally. The target voltage value is the closest one to the voltage level nominal voltage among those generators.
 
 ### ShuntCompensatorModification
 <span style="color: red">TODO</span>