DOC: (mpfr) Improve example a bit

Author: seberg

mpfdtype/README.md

@@ -5,19 +5,29 @@ must be linked with are not.
 
 A very basic example::
 
-    import mpfdtype as mpf
+    import numpy as np
+    from mpfdtype import MPFDType, MPFloat
 
     # create an array with 200 bits precision:
-    arr = np.arange(10).astype(MPFDType(200))
+    arr = np.arange(3).astype(MPFDType(200))
+    print(repr(arr))
+    # array(['0E+00', '1.0E+00', '2.0E+00'], dtype=MPFDType(200))
 
-    # Math uses the input precision:
-    arr**2 + np.sqrt(arr)
+    # Math uses the input precision (wraps almost all math functions):
+    res = arr**2 + np.sqrt(arr)
+    print(repr(res))
+    # array(['0E+00', '2.0E+00',
+    #        '5.4142135623730950488016887242096980785696718753769480731766784E+00'],
+    #       dtype=MPFDType(200))
 
     # cast to a different precision:
-    arr2 = arr.astype(mpf.MPFDtype(500))
+    arr2 = arr.astype(MPFDType(500))
+    print(repr(arr2))
+    # array(['0E+00', '1.0E+00', '2.0E+00'], dtype=MPFDType(500))
 
-    res = arr1 + arr2
-    print(res)  # uses the larger precision now
+    res = arr + arr2
+    print(repr(res))  # uses the larger precision now
+    # array(['0E+00', '2.0E+00', '4.0E+00'], dtype=MPFDType(500))
 
 There also is an `mpf.MPFloat(value, prec=None)`.  There is no "context"
 as most libraries like this (including mpfr itself) typically have.