Make gravity units more explicit (#421)

Add admonitions in forward modelling functions for prisms and
tesseroids, making the returned units more explicit.

harmonica/_forward/prism.py

@@ -60,7 +60,7 @@ def prism_gravity(
     progressbar=False,
     disable_checks=False,
 ):
-    """
+    r"""
     Gravitational fields of right-rectangular prisms in Cartesian coordinates
 
     Compute the gravitational potential, gravitational acceleration and tensor
@@ -76,6 +76,14 @@ def prism_gravity(
         i.e. the ``g_ez`` is the non-diagonal **easting-downward** tensor
         component.
 
+    .. important::
+
+        - The gravitational potential is returned in
+          :math:`\text{J}/\text{kg}`.
+        - The gravity acceleration components are returned in mgal
+          (:math:`\text{m}/\text{s}^2`).
+        - The tensor components are returned in Eotvos (:math:`\text{s}^{-2}`).
+
     Parameters
     ----------
     coordinates : list of arrays
@@ -124,6 +132,8 @@ def prism_gravity(
     -------
     result : array
         Gravitational field generated by the prisms on the computation points.
+        Gravitational potential is returned in :math:`\text{J}/\text{kg}`,
+        acceleration components in mgal, and tensor components in Eotvos.
 
     Notes
     -----

harmonica/_forward/tesseroid.py

@@ -41,7 +41,7 @@ def tesseroid_gravity(
     dtype=np.float64,
     disable_checks=False,
 ):
-    """
+    r"""
     Compute gravitational field of tesseroids on computation points.
 
     .. warning::
@@ -51,6 +51,13 @@ def tesseroid_gravity(
         It is equivalent to the opposite of the radial component, therefore
         it's positive if the acceleration vector points inside the spheroid.
 
+    .. important::
+
+        - The gravitational potential is returned in
+          :math:`\text{J}/\text{kg}`.
+        - The gravity acceleration components are returned in mgal
+          (:math:`\text{m}/\text{s}^2`).
+
     Parameters
     ----------
     coordinates : list of arrays
@@ -102,7 +109,8 @@ def tesseroid_gravity(
     -------
     result : array
         Gravitational field generated by the tesseroids on the computation
-        points.
+        points. Gravitational potential is returned in
+        :math:`\text{J}/\text{kg}` and acceleration components in mgal.
 
     References
     ----------