doc: refactor signal info in child_process.md
requested to merge github/fork/RaisinTen/doc/add-signal-to-exec-options-in-child_process.md into master
- Since
exec
callsexecFile
andexecFile
internally callsspawn
withoptions.signal
, thesignal
parameter has been documented underexec
as well. - Refactor the description of
signal
under all the functions. - Add examples of usage of
signal
under all the functions and add missingrequire
s in the other examples.