DOC, MAINT: Spell check and break long lines in ufunc_docstrings.py.

Some bits are also rephrased and nan is replaced by NaN where
appropriate.
DOC: Correct and extend the documentation of fmod and remainder ufuncs.

Closes #3563.
DOC: Document ldexp and frexp.

The documentation needs to be in umathmodule.c as that is where ldexp
and frexp and defined. I moved the current documention from
add_newdocs.py to ufunc_docstrings.py, manually translated them into C
strings, and inserted them into umathmodule.c.

Closes #2354.
Merge pull request #4400 from charris/fix-ufunc-documention

Fix ufunc documention