add comprehensive docstrings

Author: vtavana

dpctl/tensor/_elementwise_funcs.py

@@ -215,7 +215,7 @@
         Default: "K".
 Returns:
     usm_narray:
-        an array containing the element-wise exponential of x.
+        an array containing the element-wise exponential.
         The data type of the returned array is determined by
         the Type Promotion Rules.
 """
@@ -308,7 +308,7 @@
         Default: "K".
 Returns:
     usm_narray:
-        An array which is True where x is NaN, False otherwise.
+        An array which is True where `x` is NaN, False otherwise.
         The data type of the returned array is boolean.
 """