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 #4372 from charris/fix-gh-2423

DOC: Fix documentation of normed keyword in histogram2d and histogramdd.