Improve README and code comprehensibility

[Sidnioulz]

Hi folks,

This is a tiny PR with two commits that:

• Ensure all the delay implementations use the same vocabulary as the documented retryDelay option
• Add more details to documentation on delay handling, including noDelay and an advanced exponentialDelay example, and a Retry-Header behaviour explanation

I've taken upon to writing this because I initially did not notice that axios-retry handled these things out of the box and coded them on my end, until I stumbled upon the source code. I hope this PR will help prospective users better understand the capabilities of this plugin :)