added modpi etc. documentation. Updated the example for round() using…

… non-ten basis

doc/stdlib/base.rst

@@ -1972,6 +1972,18 @@ Mathematical Operators
 
    Modulus after division, returning in the range [0,m)
 
+.. function:: modpi(x)
+
+   Modulus after division by pi, returning in the range [0,pi). More accurate than mod(x,pi).
+
+.. function:: mod2pi(x)
+
+   Modulus after division by 2pi, returning in the range [0,2pi). More accurate than mod(x,2pi).
+
+.. function:: modpio2(x)
+
+   Modulus after division by pi/2, returning in the range [0,pi/2). More accurate than mod(x,pi/2).
+
 .. function:: rem(x, m)
 
    Remainder after division
@@ -2468,7 +2480,7 @@ Mathematical Functions
 
 .. function:: round(x, [digits, [base]])
 
-   ``round(x)`` returns the nearest integral value of the same type as ``x`` to ``x``. ``round(x, digits)`` rounds to the specified number of digits after the decimal place, or before if negative, e.g., ``round(pi,2)`` is ``3.14``. ``round(x, digits, base)`` rounds using a different base, defaulting to 10, e.g., ``round(pi, 3, 2)`` is ``3.125``.
+   ``round(x)`` returns the nearest integral value of the same type as ``x`` to ``x``. ``round(x, digits)`` rounds to the specified number of digits after the decimal place, or before if negative, e.g., ``round(pi,2)`` is ``3.14``. ``round(x, digits, base)`` rounds using a different base, defaulting to 10, e.g., ``round(pi, 1, 8)`` is ``3.125``.
 
 .. function:: ceil(x, [digits, [base]])