Docs: improve description of psql's %R prompt escape sequence.
Docs: improve description of psql's %R prompt escape sequence.

Dilian Palauzov pointed out in bug #14201 that the docs failed to mention
the possibility of %R producing '(' due to an unmatched parenthesis.

He proposed just adding that in the same style as the other options were
listed; but it seemed to me that the sentence was already nearly
unintelligible, so I rewrote it a bit more extensively.

Report: <adbc935ecdc997096f7139dab654c07e0df4d5a2@wrigleys.postgresql.org>

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