doc: document argument variant in the repl.md

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc, repl

Description of change

Document that options in the repl.start([options]) can be a string.

This commit is separated from #10160 on demand for more careful evaluation.

Please, consider these facts:

1. String argument is already used in the doc examples here and here.

2. It is also used in the repl.js comment example.

3. The chain from string argument to prompt and other options setting is this: exports.start -> REPLServer -> Interface in the readline.js