Document foldl and foldr implementation
Correct RST warnings
Document mapreduce and friends

Update documentation for reduce, foldl, and foldr.
Add mapreduce documentation for variant with v0.
Add mapfoldl, mapfoldr documentation.
Regenerate help.jl
Merge pull request #9027 from eschnett/mapreduce-doc

Update documentation for mapreduce, mapfoldl, mapfoldr