Update documentation based on changes to testFiles and provide examples of syntax for multi-value config options via CLI

[andrew-codes]

I'm submitting a...

[ ] Bug report
[x] Content update
[ ] Process update (build, deployment, ... )

Type of bug / changes

This [PR[(https://github.com/cypress-io/cypress/pull/5402) introduced the ability for testFiles to be mutli-value (a string or an array of strings). Please update the docs to reflect this change.

Please provide examples of config CLI options that are multi-value. There are currently no examples showing how to pass multiple values to config properties via the CLI. The supported syntax is: --config=testFiles=["**/*.js","**/*.coffee"].