The REPL's :show prints the doc of a visible name

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Co-Authored-By: eca-agent <git@eca.dev>

_02_.Writing-programs.sz

@@ -1312,4 +1312,29 @@ bjo> (g 10)
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-The same incremental compilation model applies to trait implementations (@code{impl}): an implementation entered at the prompt is visible to subsequent entries, but cannot retroactively modify calls compiled earlier. Type @code{:help} to view available REPL commands, or press @code{Ctrl-D} (or type @code{:quit}) to exit.
+The same incremental compilation model applies to trait implementations (@code{impl}): an implementation entered at the prompt is visible to subsequent entries, but cannot retroactively modify calls compiled earlier.
+
+@code{:show name} prints the documentation of a name you can use at the prompt: what it is, where it comes from, how it is called, and the text its author wrote in a @code{(:doc ...)} form. It then lists the other usable names containing the text, in any case, with their modules and one-line summaries. Only names that an entry typed now could use are considered, that is builtins, names your imports bring in, and names defined at the prompt. So @code{:show random-bool} prints nothing until @code{(std random)} is imported:
+
+@codeblock[#:lang "text"]|{
+bjo> :show random-bool
+bjo> (import (std random))
+bjo> :show random-bool
+random-bool: function from (std random)
+  (random-bool #:rng (parameter-ref current-random))
+  : (-> (#:rng RandomSource) bool)
+
+  #t or #f, equally likely.
+
+  #:rng    The source to draw from.
+  returns  One random bit. They are drawn 64 at a time.
+
+  See also: random-chance?
+bjo> :show random-u
+  random-u32  (std random)  32 random bits.
+  random-u64  (std random)  64 random bits.
+}|
+
+Import modifiers apply as they would in an entry: after @code{(import (prefix (std random) "r/"))} the name to ask about is @code{r/random-bool}, and @code{:show} says that it is @code{random-bool} from @code{(std random)}. A name with no documentation is shown with where it comes from and "with no doc".
+
+Type @code{:help} to view available REPL commands, or press @code{Ctrl-D} (or type @code{:quit}) to exit.