Clarify doc for "norm" kwarg to `imshow`.

[anntzer]

"norm" also applies when X is a 2D-array of integers -- i.e, similarly
to the "cmap" kwarg. Make the docs for the two kwargs consistent.

[efiring]

There is more that needs to be fixed in the imshow docstring. It is not taking into account that MxN can be an integer array if norm is NoNorm. Also the "luminance (grayscale...)" is confusing, since it is generally not intended to indicate gray, and it is not really luminance--it is simply the input to the (norm, cmap) combination.

[anntzer]

(If you have specific edits to suggest, please post them as diffs (or PR to my repo but that's a bit overkill).)

[efiring]

Here's a diff for _axes.imshow:
imshow_Xdoc_diff.txt
Of course, looking down a little farther one sees no end of other docstrings needing work.
It would be nice to be able to factor out some of the common chunks, such as the descriptions of cmap and norm.

[tacaswell]

backported to v2.x as 4c4ead5