doc: `process` formatting issues

[mscdex]

• Version: master, v6.x
• Platform: n/a
• Subsystem: doc

I noticed there are some markdown formatting issues with the process documentation:

• There is a missing backtick in the description for process.setuid() after process.setuid(id).
• There are some missing link references: here (process.exitCode), here (process.exitCode), and here (LTS).
• In the process.release section, there are some instances of markdown underscores for italicization showing up literally in file names/extensions: _.tar.gz_ (2) and _node.lib_ (1). I think if the underscores are moved right outside the backticks, the italics should render correctly.