Language/description updates for stock missions

Hey there, I just flew through the first few of the stock missions and ended up having to repeat a few of them since the descriptions weren't entirely clear.  I went through the mission files and updated them with new descriptions based on the mission specs in the files, just wanted to offer you the update in case it might help others!

I also added a requirements for antennae to the Mun I mission since it kind of made the mission a little easier to explain and didn't really change the difficulty.