Update docs for img_as_ubyte

The line in `img_as_ubyte` needs correction; ubyte cannot hold negative data. From a quick experiment it seems like both `img_as_uint` and `img_as_ubyte` clip negative values (and not shift them to the positive domain).
Added a short paragraph + see also section in docstring of
exposure.equalize_adapthist
Fixed broken link to NumPy's docstring standard.
Merge pull request #1230 from emmanuelle/clahe

Update CLAHE docstring, and fix numpy docstring conventions link.
Protect exposure.histogram from integer overflow

Fixes #1228 .

I also removed a min() calculation and prevented an image copy when
offset is 0, which is most of the time.
Merge pull request #1232 from jni/negative-histogram

Protect exposure.histogram from integer overflow
Update docs in dtype.py: mention scaling of values
Merge pull request #1229 from almarklein/patch-1

Update docs for img_as_ubyte
fixed blob area overlap function warning
Removed deprecated module name filter from tutorial
Add fused type specialization for moments functions
Use uint8 type for simple moments functions
Merge pull request #1235 from vighneshbirodkar/blob_fix

Fix blob area overlap function warning

Fixes #1233
Merge pull request #1237 from emmanuelle/filters_rank

Remove deprecated module name `filter` from tutorial
Add encoding header
Merge pull request #1239 from ahojnnes/moments-templ

Moments (regionprops) speedup
Add signed edge operators and deprecate unsigned versions
Add deprecation instructions for old unsigned edge map versions
Use new edge operator functions in test cases
Add missing comma