Update astensor.md

docs/api/functions/astensor.md

@@ -10,7 +10,47 @@ grand_parent: API Reference
 
 Converts a `Tensor2s` or `Tensor4s` data type to a full tensor storage `Tensor2` or `Tensor4`.
 
-- `astensor(T)`
+\boldsymbol{C} = \begin{bmatrix}
+    C_{11} & C_{22} & C_{33} & C_{12} & C_{23} & C_{13}
+\end{bmatrix}^T \longrightarrow
+%
+\boldsymbol{C} = \begin{bmatrix}
+    C_{11} & C_{12} & C_{13} \\
+    C_{12} & C_{22} & C_{23} \\
+    C_{13} & C_{23} & C_{33}
+\end{bmatrix} \qquad
+
+\mathbb{A} = \begin{bmatrix}
+    A_{1111} & A_{1122} & A_{1133} & A_{1112} & A_{1123} & A_{1113} \\
+    A_{2211} & A_{2222} & A_{2233} & A_{2212} & A_{2223} & A_{2213} \\
+     \dots   &  \dots   &  \dots   &  \dots   &  \dots   &  \dots   \\
+    A_{1311} & A_{1322} & A_{1333} & A_{1312} & A_{1323} & A_{1313}
+\end{bmatrix}
+
+\longrightarrow \begin{bmatrix}
+    A_{1111} & A_{1112} & A_{1113} &
+    A_{1121} & A_{1122} & A_{1123} &
+    A_{1131} & A_{1132} & A_{1133} \\
+    %
+    A_{1211} & A_{1212} & A_{1213} &
+    A_{1221} & A_{1222} & A_{1223} &
+    A_{1231} & A_{1232} & A_{1233} \\
+    %
+    \dots & \dots & \dots & \dots & \dots & \dots & \dots & \dots & \dots \\
+    A_{3111} & A_{3112} & A_{3113} &
+    A_{3121} & A_{3122} & A_{3123} &
+    A_{3131} & A_{3132} & A_{3133}
+    %
+\end{bmatrix} \qquad
 
 Alias:
-- `tensorstore(T)`
+- `tensorstore(T)`
+
+### Example
+
+```fortran
+type(Tensor2s) :: T
+type(Tensor2) :: U
+
+U = astensor(T)
+```