Clarify the tsconfig documentation

[JoshStern]

Hey! I was trying to understand if options set under tsconfig were applied as overrides of the found tsconfig.json or would replace it entirely.

// jest.config.js
module.exports = {
  // ...
  globals: {
    'ts-jest': {
      // Behavior of this section
      tsconfig: {
        sourceMap: true
      }
    }
  }
};

From this section it looks like it's an override:

ts-jest/src/config/config-set.ts

Lines 523 to 546 in cd9ff8c

	let config = { compilerOptions: Object.create(null) }
	let basePath = normalizeSlashes(this.rootDir)
	const ts = this.compilerModule
	// Read project configuration when available.
	const configFileName: string | undefined = resolvedConfigFile
	? normalizeSlashes(resolvedConfigFile)
	: ts.findConfigFile(normalizeSlashes(this.rootDir), ts.sys.fileExists)
	if (configFileName) {
	this.logger.debug({ tsConfigFileName: configFileName }, 'readTsConfig(): reading', configFileName)
	const result = ts.readConfigFile(configFileName, ts.sys.readFile)
	// Return diagnostics.
	if (result.error) {
	return { errors: [result.error], fileNames: [], options: {} }
	}
	config = result.config
	basePath = normalizeSlashes(dirname(configFileName))
	}
	// Override default configuration options `ts-jest` requires.
	config.compilerOptions = {
	...config.compilerOptions,
	...compilerOptions,
	}

Is that correct? And if so, can I open a PR to make it clearer in the docs here?

I searched the docs pretty thoroughly but if it's already there and I missed it, sorry!