docs: Mention import foo {.all.} syntax

Mention the `import foo {.all.}` syntax in the manual,
with a caveat about private imports.
Also link to the experimental importutils module.

doc/manual.rst

@@ -6350,6 +6350,12 @@ does not export these identifiers.
 
 The `import` statement is only allowed at the top level.
 
+In some situations, it may be useful to import all symbols (public or private)
+from a module. The syntax `import foo {.all.}` can be used to import all
+symbols from the module `foo`. Note that importing private symbols is
+generally not recommended.
+
+See also the experimental `importutils <importutils.html>`_ module.
 
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