doc: need better link to signals list in process documentation

[mscdex]

• Version: all
• Platform: n/a
• Subsystem: doc

In the 'Signal Events' section of the process documentation, there is a link to a man page that supposedly lists the names of the various signals that can be listened for. However, the page that is currently linked (sigaction(2)) doesn't show all of the valid signals and the ones that it does show are not formatted very well. I think a better link would be to signal(7), which does list all of the signal names and in an easy-to-spot/read format.