Update pattern logger precision example and description

[cretzbach]

The description of the logger precision using periods was confusing and the examples were wrong. This commit fixes the description, the wrong examples and adds one more example for clarification.

I was confused by the examples and after trying them in my own implementation it turns out that using periods in the precision pattern for the logger is straight forward but differs from the examples documented here. I did not test every precision value, only periods with asteri

Checklist

• Base your changes on 2.x branch if you are targeting Log4j 2; use main otherwise
• ./mvnw verify succeeds (if it fails due to code formatting issues reported by Spotless, simply run ./mvnw spotless:apply and retry)
• Non-trivial changes contain an entry file in the src/changelog/.2.x.x directory
• Tests for the changes are provided
• Commits are signed (optional, but highly recommended)