[Rust] Improve documentation for StringDictionaryBuilder

[asfimport]

I find myself trying to remember the exact incantation to create a StringDictionaryBuilder so it should be a doc example

Reporter: Andrew Lamb / @alamb
Assignee: Andrew Lamb / @alamb

PRs and other links:

• GitHub Pull Request #9169

_{Note: This issue was originally created as ARROW-11216. Please see the migration documentation for further details.}

[asfimport]

Jorge Leitão / @jorgecarleitao:
Issue resolved by pull request 9169
#9169