Update documentation for default value in Backoff.exponential where multiplier is not specified

[roderick675]

Motivation:

Docs were confusing when using the framework. Needed to dig into the code to find the default multipler. So let's add it in the documentation

[CLAassistant]

All committers have signed the CLA.

[github-actions]

🔍 Build Scan® (commit: 3b2ea7c)

Job name	Status	Build Scan®
build-ubicloud-standard-8-jdk-8	✅	https://ge.armeria.dev/s/ab5eatom4xb24
build-ubicloud-standard-8-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/v36lbhmpjvnd2
build-ubicloud-standard-8-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/7avjlaeqrupes
build-ubicloud-standard-8-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/f7we2ualtp2vs
build-ubicloud-standard-8-jdk-17-leak	❌ (failure)	https://ge.armeria.dev/s/5gbb7jfuwvimk
build-ubicloud-standard-8-jdk-11	✅	https://ge.armeria.dev/s/e52lijilxdmlo
build-macos-12-jdk-21	✅	https://ge.armeria.dev/s/o43esnp5uhlbi

[jrhee17]

👍 👍

[ikhoon]

👍👍

[ikhoon]

@roderick675 Would you fix the checkstyle violation?

[roderick675]

@ikhoon done!

[minwoox]

Thanks!

[ikhoon]

Thanks!