build: fixes for ns3 run

This MR includes the following changes:

• renames ./ns3 --list to ./ns3 show targets
• changes the help message of ./ns3 run to mention ./ns3 show targets, which prints some of the available targets to build/run
• changes the help message of ./ns3 run to include different ways of passing arguments to the target that will be run
• fixes wrong ways of using ./ns3 run that were found in the docs (e.g. ./ns3 run="program")

Fixes #687 (closed)