Update overriding env variables wording to be clearer

[jennifer-shehane]

Values in here will overwrite conflicting values in cypress.json.

From discussion on cypress-io/cypress#909, this should really say:

Values in here will overwrite conflicting environment variables in cypress.json.

[clairecliu]

I would like to give this a try.

[danverde]

The docs currently say:

You can also use environment variables to override configuration values. This is especially useful in Continuous Integration or when working locally. This gives you the ability to change configuration options without modifying any code or build scripts.

By default, any environment variable that matches a corresponding configuration key will override the cypress.json value.

and

Environment variables that do not match configuration keys will instead be set as regular environment variables

From what I understood of cypress-io/cypress#909 environment variables cannot override configuration values and the docs should be updated accordingly