Change documentation for reduce

[Inkrementator]

• Document the function signatures for the passed in functions at the most obvious place (in the past, it was often faster for me to try this out instead of searching it in the docs)
• paragraphs
• equalize fold/reduce documentation a bit

I also changed predicate to function, which is slightly more correct, but actually should be callable. I think function is easier to understand for newcomers to the language while everybody else probably understands from context that the alias can be any callable, but reviewers might object here.

[dlang-bot]

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