Update localSchemaFile docs in "configuring apollo"

[JakeDawkins]

The apollo config now supports using multiple localSchemaFiles in the config file!

This PR updates the docs for configuring apollo projects to reflect that.

Note: the name is purposefully localSchemaFile and not localSchemaFiles to support backwards-compatibility and to avoid adding yet another option and CLI flag.

[JakeDawkins]

@StephenBarlow for things like this, how should we go about documenting or noting that this won't work with older versions of the config used in old CLI releases or old VS code releases?

We added this support in a patch version (probably an oversight there 😬) of both the extension and CLI ([email protected], [email protected])

[StephenBarlow]

@JakeDawkins Regarding version availability, I recommend a little blockquote notice immediately after the feature in question is mentioned, such as:

> Support for linking _multiple_ local schema files is available in the Apollo CLI version 2.18.2 and later, and in the Apollo VS Code extension version 1.10.2 and later. 

After enough time has passed that most folks are probably on these versions or later, we can remove the notice.

[GuillaumeDesforges]

Had to dig a lot to find this. Hope it will be merged soon.