FAQ entry on calling un-patched function inside patch

[kleinschmidt]

I've added an FAQ entry on how to call the "un-patched" version of a function inside a patch (if you e.g. want to modify the arguments before calling a function).

@omus mentioned that using @mock inside a patch can be useful as well but I couldn't immediately come up with an example off the top of my head, so I just left a short note about that being possible.

[omus]

Here's probably a more useful example:

Suggested change

	Simply call the patched function without `@mock`:
	```julia
	julia> f(x) = x + 1
	f (generic function with 1 method)
	julia> g(x) = @mock(f(x)) * 2
	g (generic function with 1 method)
	julia> fp = @patch f(x) = f(-x)
	Patch{typeof(f)}(f, var"##f_patch#240")
	julia> g(3)
	8 # = (3 + 1) * 2
	julia> apply(fp) do
	g(3)
	end
	-4 # = (-3 + 1) * 2
	```
	Simply call the function without using `@mock` within the patch. For example we can count the number of calls a recursive function does like this:
	```julia
	function fibonacci(n)
	if n <= 1
	return n
	else
	return @mock(fibonacci(n - 1)) + @mock(fibonacci(n - 2))
	end
	end
	calls = Ref(0)
	p = @patch function fibonacci(n)
	calls[] += 1
	return fibonacci(n) # Calls original function
	end
	apply(p) do
	@test @mock(fibonacci(1)) == 1
	@test calls[] == 1
	calls[] = 0
	@test @mock(fibonacci(4)) == 3
	@test calls[] == 9
	end
	```