missing document for utils: plugins: Provide a choice for encoding raw UTF-8 strings

[owanio1992]

feature source PR: fluent/fluent-bit#10665
document update PR: #2131

[eschabell]

Thanks @owanio1992 I'll take a look at this as the code PR fluent/fluent-bit#10665 was merged already.

[eschabell]

Waiting on @owanio1992 to fix DCO issues on the docs PR #2131

[eschabell]

Fixed by PR #2131