trying to reduce gain confusion

[Greg Troxel]

I just saw a ticket comment which reminded me that I don't understand
what's going on with gain. So I wrote a wiki page with a lot of
missing info, but tried to organize what I could piece together.

https://github.com/merbanan/rtl_433/wiki/Gain-Control

It strikes me that we should implement "-g list" which prints out the
allowable tuner gain values, and adjust the -g help to say "tuner gain".

But I could be confused. Fixes welcome, directly in the wiki of course,
here if of broad interest, direct to me for nits.

Greg