Notification: clarify _readthedocs/html path solution

[agjohnson]

If you don't output a file during the build, you will see the following notification:

However, if you try outputting to this path without using the environment variable, you'll get a path not found error:

echo "Foo" > _readthedocs/html/index.html
/bin/sh: 1: cannot create _readthedocs/html/index.html: Directory nonexistent

• Maybe note the environment variable in the error notification in addition, or instead of _readthedocs/html.
• Point to the documentation we have on this environment variable