Improve docstrings of magnetic_field functions

Improve the description of the outputs of the `magnetic_field` functions
for dipoles and prisms. List each element of the tuple that is returned
as separate returns.

choclo/dipole/_forward.py

@@ -61,11 +61,15 @@ def magnetic_field(
 
     Returns
     -------
-    b : array
-        Array containing the three components of the magnetic field generated
-        by the dipole on the observation point in :math:`\text{T}`.
-        The components are returned in the following order: ``b_e``, ``b_n``,
-        ``b_u``.
+    b_e : float
+        Easting component of the magnetic field on the observation point in
+        :math:`\text{T}`.
+    b_n : float
+        Northing component of the magnetic field on the observation point in
+        :math:`\text{T}`.
+    b_u : float
+        Upward component of the magnetic field on the observation point in
+        :math:`\text{T}`.
 
     Notes
     -----

choclo/prism/_magnetic.py

@@ -81,13 +81,21 @@ def magnetic_field(
 
     Returns
     -------
-    b_e, b_n, b_u : floats
-        The three components of the magnetic field generated by the prism on
-        the observation point in :math:`\text{T}`.
-        The components are returned in the following order: ``b_e``, ``b_n``,
-        ``b_u``.
-        Return a tuple full of ``numpy.nan`` if the observation point falls in
-        a singular point: prism vertices, prism edges or interior points.
+    b_e : float
+        Easting component of the magnetic field on the observation point in
+        :math:`\text{T}`.
+        It will be ``numpy.nan`` if the observation point falls in a singular
+        point: prism vertices, prism edges or interior points.
+    b_n : float
+        Northing component of the magnetic field on the observation point in
+        :math:`\text{T}`.
+        It will be ``numpy.nan`` if the observation point falls in a singular
+        point: prism vertices, prism edges or interior points.
+    b_u : float
+        Upward component of the magnetic field on the observation point in
+        :math:`\text{T}`.
+        It will be ``numpy.nan`` if the observation point falls in a singular
+        point: prism vertices, prism edges or interior points.
 
     Notes
     -----