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.
DOC: Improve the documentation of numpy.load.

Add the fact that for pickled files, a file-like object must also
support the readline() method.

Closes #2807.
DOC: Fix documentation of normed in histogram2d and histogramdd.

The documentation misrepresented what happened, leaving out division
by the total number of sample points.

Also run spellcheck over function_base.py and twodim_base.py and break
some long lines.

Closes #2423.
Merge pull request #4406 from charris/backport-1.8-doc-fixes

Backport 1.8 doc fixes