Ensure --doctool is run from root directory

[RedMser]

There are two ways to run doctool:

• --doctool (uses cwd by setting . as the path)
• --doctool <path> (absolute or relative path)

While convenient, the former method implicitly relies on the cwd. So it might end up generating doc, modules and platform directories inside of your build directory or home directory.
Now we check for the existence of the doc folder for this case only, to avoid accidents.

This is technically a breaking change, but I assume most scripted usage of --doctool either runs in the repository root already, or specifies a path in the command line.

While we could also check for the doc folder in the case of an explicit path being passed, it might be desired to generate the XML structure outside of the repository folder for other purposes (such as a simple to parse list of classes, members, etc.)?

[akien-mga]

Thanks!