[doc] missing data index value in KDTree, cKDTree

[Carreau]

cKDtree says Missing neighbors are indicated with self.n. though self.n is not documented (AFAICT as being len(data)

KDTree does not mention anything about missing indices, but seem to do the same.

[rgommers]

Documenting n as a public attribute would be good. And also checking for other missing attributes in the docs.

[gfyoung]

Ignore that commit. I meant to address #5751. Can look at this one in a separate PR though.

[goelakash]

I would like to work on this. So:

1. adding a mention of n as equivalent to size of tree in attribute list in both kdtree,py and ckdtree.pyx
2. adding "Missing parameters are indicated by self.n" in KDTree.py

anything else that should/shouldn't be done?

[rgommers]

@goelakash sure, you're welcome to work on this. Don't bother with KDTree though, only cKDTree - we're going to be deprecating the former. I think what's needed here is adding a list of all public attributes in the main cKDTree docstring with an explanation of what they're for. See https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt#class-docstring for theAttributes`` format.

[rgommers]

Fixed in gh-6939, thanks all.