Improve ErrInvalidKeyType error messages

[Laurin-Notemann]

This small PR adds

• a little bit more context to the ErrInvalidKeyType error message "key is of invalid go type (should be []byte())"
• better description of the Parse function

Generally the original error might seem obvious but for some reason it wasn't clear to me that it was specifically referring to the type in Go, I thought it was some kind of error with my jwt token and not my signing key. It seems like an easy fix of just improving the error message slightly so its clearer what is means.

Since I only looked at the example of how parse the jwt into a correct token again I was also really confused as to what "AllYourBase" even meant. Which is why I suggest to add a small comment, so its clear that this is referring to the signing key (which it only is if you read the previous examples and specifically look at the value of the signingKey).

[oxisto]

Looks good with a minor nit. Please also change the PR title and description to remove the parse docs.