[DOC] Fix small inaccuracy in broadcasting docs

During broadcasting, dimensions with size 1 can be matched against 0-sized dimensions, and in this case it's the size 1 dimension that will be shrunk away to nothingness. So it's wrong to say that the *smaller* dimension is the one that changes.
Merge pull request #4362 from njsmith/master

[DOC] Fix small inaccuracy in broadcasting docs
ENH: tril and triu broadcasting
DOC: triu/tril broadcasting release notes
Merge pull request #4364 from argriffing/triu-broadcasting

ENH: tril and triu broadcasting