Apply suggestions from code review

Co-authored-by: Jerry Guo <6221579+Jerry-Jinfeng-Guo@users.noreply.github.com>
Signed-off-by: Tony Xiang <19280867+TonyXiang8787@users.noreply.github.com>

docs/user_manual/calculations.md

@@ -57,10 +57,10 @@ Output:
 - Power flow through branches
 - Deviation between measurement values and estimated state
 
-In order to perform a state estimation, the system should be observable. If the system is not observable, the calculation will either raise a not observable error or 
-a sparse matrix error.
+In order to perform a state estimation, the system should be observable. If the system is not observable, the calculation will raise either a `NotObservableError` or 
+a `SparseMatrixError`.
 In short, meeting the requirement of observability indicates that the system is either an overdetermined system (when the number of measurements is larger than the number of
-unknowns) or a determined system (when the number of measurements is equal to the number of unknowns). For each node, there are two unknowns, `u` and `u_angle`, so the following conditions should be met:
+... For each node, there are two unknowns, `u` and `u_angle`. Due to the relative nature of `u_angle` (relevant only in systems with at least two nodes), in total the following conditions should be met:
 
 $$
     \begin{eqnarray}
@@ -98,7 +98,7 @@ In observable systems this helps better outputting correct results. On the other
 
 ##### Necessary observability condition
 
-Based on the requirements of observability mentioned above, user needs to satisfy  at least the following conditions for state estimation calculation in `power-grid-model`.
+Based on the requirements of observability mentioned above, users need to satisfy at least the following conditions for state estimation calculation in `power-grid-model`.
 
 - `n_voltage_sensor >= 1`
 - If no voltage phasor sensors are available, then the following conditions should be satisfied:  `n_unique_power_sensor >= n_bus - 1`. Otherwise: `n_unique_power_sensor + n_voltage_sensor_with_phasor >= n_bus`
@@ -118,7 +118,7 @@ It is rather complicated to do a full sufficient and necessary observability che
 1. The system is a radial network.
 2. The system does not have voltage phasor measurements.
 
-In this case, the validation of the independent measurements is rather straightforward. If the system is not observable, the calculation will raise a not observable error instead of sparse matrix error.
+In this case, the validation of the independent measurements is rather straightforward. If the system is not observable, the calculation will raise a `NotObservableError` instead of `SparseMatrixError`.
 
 #### Short circuit calculations