comments only, explain the different ONE_AT_A_TIME algos

Seeing 3 hash algos with the same name is confusing. If they are the same
name ("one at a time"), then why are there 3 different ones? What are the
differences (without reading the code)? What are the pros and cons of each?
INSTALL lists the ONE_AT_A_TIME algos as existing, but doesn't explain why
there are 3 of them. Patch with jkeenan suggestions.