docs: Correct the documentation for math.frexp.

docs/library/math.rst

@@ -90,7 +90,10 @@ Functions
 
 .. function:: frexp(x)
 
-   Converts a floating-point number to fractional and integral components.
+   Decomposes a floating-point number into its mantissa and exponent.
+   The returned value is the tuple ``(m, e)`` such that ``x == m * 2**e``
+   exactly.  If ``x == 0`` then the function returns ``(0.0, 0)``, otherwise
+   the relation ``0.5 <= abs(m) < 1`` holds.
 
 .. function:: gamma(x)