docs: restore information about the YARN_RC_FILENAME environment variable

[romannovojilov]

What's the problem this PR addresses?

I found that starting from version 4.0.0, information about the YARN_RC_FILENAME environment variable is missing from the documentation. However, this environment variable is still supported.

Proof:

berry/packages/yarnpkg-core/sources/Configuration.ts

Lines 980 to 988 in c6764b2

	function getRcFilename() {
	const rcKey = `${ENVIRONMENT_PREFIX}rc_filename`;
	for (const [key, value] of Object.entries(process.env))
	if (key.toLowerCase() === rcKey && typeof value === `string`)
	return value as Filename;
	return DEFAULT_RC_FILENAME as Filename;
	}

How did you fix it?

I have updated the documentation to restore information about the YARN_RC_FILENAME environment variable

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.