DOC: Improvements to children_ attribute in AgglomerativeClustering
Borrow a bit from scipy definition
Small typo fix in the comments: "whther" --> "whether"
Merge pull request #3989 from hammer/patch-2

[DOC] Small typo fix in the comments: "whther" --> "whether"
Merge pull request #3976 from MechCoder/agglomerative_playground2

[MRG] DOC: Minor Improvement to `children_ attribute` in AgglomerativeClustering