I suggest changing the main docs to match the existing docstring, "Return e raised to the power of x."  

The exp() function is a thin wrapper around the C math library and where it is documented as "compute e (the base of natural logarithms) raised to the power x" or "e raised to the power X (where e is the base of the natural system of logarithms, approximately 2.71828)."  Our docs shouldn't make more or fewer promises than the upstream libraries are making.

Perhaps there can be a general note about reading too much into the math module implementation details.  We expect some relationships to only be approximate: log(x)+1≈log1p(x), log2(x)≈log(x,2.0), exp(lgamma(x))≈gamma(x), sqrt(x)≈x**0.5, etc.  These are floating point math library "facts of life".

* http://www.slac.stanford.edu/comp/unix/package/rtems/doc/html/libm/libm.info.exp.html

* https://www.gnu.org/software/libc/manual/html_node/Exponents-and-Logarithms.html