doc: child_process example for special chars

Updates the Child Process Exec function documentation providing
further information with regards to treating special characters in
the command string function argument. Note that the exec string is
processed by the shell. Updates description to show that special
shell characters vary based on the shell with a link to the Wikipedia
page regarding different command line interpreters and their
associated operating systems. Updates example code to reside in
separate lines using a codeblock and utilizes examples to comply with
code standards by using single quotes.

PR-URL: https://github.com/nodejs/node/pull/10921/
Fixes: https://github.com/nodejs/node/issues/6803


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>