Update redshell documentation

[zwetan]

The Redtamarin shell is the main executable produced by the Redtamarin C++ sources.

Originally avmshell we made it "our own" by renaming it redshell.

We have 3 versions

• redshell for the Redtamarin shell release build
• redshell_d for the Redtamarin shell debug build
• redshell_dd for the Redtamarin shell debug-debugger build

Because the different versions have different command-line options
and because those are "our main binaries" we should apply the same documentation logic
same as #189