Currently, the second paragraph of docs starts with "The arguments must have numeric types." That seems to be too restrictive and many third-party packages already break this rule. E.g. SymPy support arbitrary symbolic expressions:

In [1]: pow(x, y)
Out[1]: 
 y
x 

In [2]: pow(x, y, z)
Out[2]: 
 y      
x  mod z

Diofant also uses both forms for polynomial arithmetic:

In [1]: R, x = ring('x', QQ)

In [2]: pow(x + 1, 7, 2*x**2 - 1)
Out[2]: 239/8*x + 169/8

In [3]: ((x + 1)**7) % (2*x**2 - 1)
Out[3]: 239/8*x + 169/8

In [4]: (x + 1)**2
Out[4]: x**2 + 2*x + 1

In [5]: pow(x + 1, 2)
Out[5]: x**2 + 2*x + 1

I suggest to drop this requirement. In fact, first paragraph already mention equivalents for two-arg and three-arg versions of the pow(). We can rephrase it like this: "Return base to the power exp, an equivalent to base**exp; if mod is present, return base to the power exp, modulo mod, an equivalent to (base**exp) % mod (but computed more efficiently)."

The third paragraph also might instead specify requirements for the mod in case of integer base and exponent, not just for builtin int's.