Some clarifications for fork(2) and _Exit(3) man pages

[kib]

Summary

fork(2): comment about doubtful use of stdio and exit(3) in example
Add fflush(stdout) as the common idiom. Explain the need to use exit() but advise against it.

_Exit(3): document implementation

[emaste]

lib/libc/sys/fork.2

178

Maybe s/at least/and other possible issues/ or similar, if I understand correctly.

[kib]

reformulate 'at least'.

[markj]

lib/libc/stdlib/exit.3

71

I do not understand what this is referring to: tmpfile() unlinks the newly created file before returning.

84

Assuming this refers to tmpfile(3), I do not see why.

[kib]

Remove references to tmpfile, add notion about destructors registered with __cxa_atexit(3)