Specify numeric arrays for argmin and argmax (#342)

Author: honno

spec/API_specification/searching_functions.md

@@ -25,7 +25,7 @@ Returns the indices of the maximum values along a specified axis. When the maxim
 
 -   **x**: _<array>_
 
-    -   input array.
+    -   input array. Should have a numeric data type.
 
 -   **axis**: _Optional\[ int ]_
 
@@ -50,7 +50,7 @@ Returns the indices of the minimum values along a specified axis. When the minim
 
 -   **x**: _<array>_
 
-    -   input array.
+    -   input array. Should have a numeric data type.
 
 -   **axis**: _Optional\[ int ]_