[CMake] Please update the documentation for execute_process

[Jeffrey Walton]

The documentation for execute_process has some room for improvement.

We recently got burned by a problem that has existed since at least
2011: https://stackoverflow.com/q/6797395/608639 . Once we learned the
problem we could research a bit. The problem cost us over 8 man hours
when it should not have been a problem in the first place. Considering
there's nothing special about us, it has probably wasted thousands of
man hours over the years.

Here are the actionable items for the execute_process documentation task:

 * please clearly state the first argument is the command only, and
not command + arguments
 * please clearly state whether arguments need to be quoted
 * please clearly state whether arguments need to be comma separated
 * please provide an example (or examples) to show how to specify
multiple arguments for a command
 * please provide an example (or examples) to show how to redirect
output when using a command

Below is a typical case for us
(https://github.com/weidai11/cryptopp/blob/master/CMakeLists.txt). If
it looks pretty shitty, it probably is. That's the best we have been
able to come up with based on the documentation.

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