docs: Import documentation from source-code inline comments.

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+:mod:`cmath` --- mathematical functions for complex numbers
+===========================================================
+
+.. module:: cmath
+   :synopsis: mathematical functions for complex numbers
+
+The ``cmath`` module provides some basic mathematical funtions for
+working with complex numbers.
+
+
+Functions
+---------
+
+.. function:: cos(z)
+
+   Return the cosine of ``z``.
+
+.. function:: exp(z)
+
+   Return the exponential of ``z``.
+
+.. function:: log(z)
+
+   Return the natural logarithm of ``z``.  The branch cut is along the negative real axis.
+
+.. function:: log10(z)
+
+   Return the base-10 logarithm of ``z``.  The branch cut is along the negative real axis.
+
+.. function:: phase(z)
+
+   Returns the phase of the number ``z``, in the range (-pi, +pi].
+
+.. function:: polar(z)
+
+   Returns, as a tuple, the polar form of ``z``.
+
+.. function:: rect(r, phi)
+
+   Returns the complex number with modulus ``r`` and phase ``phi``.
+
+.. function:: sin(z)
+
+   Return the sine of ``z``.
+
+.. function:: sqrt(z)
+
+   Return the square-root of ``z``.
+
+
+Constants
+---------
+
+.. data:: e
+
+   base of the natural logarithm
+
+.. data:: pi
+
+   the ratio of a circle's circumference to its diameter