DOC: Min max docstrings

1. Mention the 4 variants of min/max functions in each other's See Also sections, clarifying how they differ
2. Don't waste space describing both the min and max in the See Also, just list the name of the opposite function
3. Add note about not using amax for pairwise comparisons
4. Diffed each min with the corresponding max function, and updated each with any changes that had been made to the other
5. Remove `fmin(x1, x2[, out])` from docstring

fmax and fmin are not included on the web docs, though.  Can they be added to a file (numpy/doc/source/reference/routines.math.rst?) to make their docstrings show up?