[std.algorithm] Improve fold docs

[ntrel]

Mention iteratively calling predicates in summary.
Use list to explain calling a single predicate with a seed.
Mention multiple results can be produced in description.
Use list for see also.
Rename seed parameter seeds and fix docs.
Fix result docs when >1 predicate.

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[pbackus]

Should reduce be updated too?

[ntrel]

@pbackus Maybe we can get rid of reduce in std2?

[pbackus]

@ntrel I hope so, but we're still a long way from std2, and we should probably avoid having two out-of-sync descriptions of the same function in the meantime. If you don't want to repeat the whole thing, maybe we could replace reduce's description with "See fold" and a link?