(doc next) doesn't mention fiber

[iacore]

next can be used on fiber, but the doc text doesn't mention that.

❯ janet
Janet 1.32.1-1ccd544b linux/x64/gcc - '(doc)' for help
repl:1:> (doc next)

    function

    (next ds &opt key)

    Gets the next key in a data structure. Can be used to iterate 
    through the keys of a data structure in an unspecified order. Keys 
    are guaranteed to be seen only once per iteration if they data 
    structure is not mutated during iteration. If key is nil, next 
    returns the first key. If next returns nil, there are no more keys 
    to iterate through.

nil
repl:3:> (def a (file/lines (file/open "/etc/passwd")))
<fiber 0x555844673640>
repl:4:> (next a nil)
0
repl:5:> (next a 0)
0
repl:6:> (next a 1)
0
repl:7:> (next a 2)
0
repl:8:> 

It says

Keys are guaranteed to be seen only once per iteration if they data structure is not mutated during iteration.

• a fiber is not commonly known as "data structure"
• the fiber self-mutates. "mutated during iteration" is closer to "mutated from the outside"

[iacore]

@bakpakin it is not completed by #1334