doc/rl-next: Add repl-and-logging

doc/manual/rl-next/repl-and-logging.md

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+---
+synopsis: "Fix chopped up repl output"
+issues: [12599]
+prs: [12604]
+---
+
+The REPL and logger now contend for standard output in a much more systematic manner.
+In particular, the synchronisation mechanism is more error-tolerant, by virtue of now using [RAII](https://en.wikipedia.org/wiki/Resource_acquisition_is_initialization).
+REPL output should no longer interrupt or be interrupted by the logger mid-message,