Document completions-detailed

* doc/emacs/help.texi (Name Help): Document completions-detailed.
* lisp/minibuffer.el (completions-detailed): Give an example in
the doc string.

doc/emacs/help.texi

@@ -319,6 +319,14 @@ variable, or a face.  If the symbol has more than one definition, like
 it has both definition as a function and as a variable, this command
 will show the documentation of all of them, one after the other.
 
+@vindex completions-detailed
+  If the @code{completions-detailed} user option is non-@code{nil},
+some commands provide details about the possible values when
+displaying completions.  For instance, @kbd{C-h o TAB} will then
+include the first line of the doc string, and will also say whether
+each symbol is a function or a variable (and so on).  Which details
+are included varies depending on the command used.
+
 @node Apropos
 @section Apropos
 @cindex apropos

etc/NEWS

@@ -174,6 +174,7 @@ completion list popped up by 'M-x' shows the key bindings for all the
 commands shown in the list of candidate completions that have a key
 binding.
 
++++
 ** New user option 'completions-detailed'.
 When non-nil, some commands like 'describe-symbol' show more detailed
 completions with more information in completion prefix and suffix.

lisp/minibuffer.el

@@ -1824,8 +1824,9 @@ in one column."
 
 (defcustom completions-detailed nil
   "When non-nil, display completions with details added as prefix/suffix.
-Some commands might provide a detailed view with more information prepended
-or appended to completions."
+This makes some commands (for instance, \\[describe-symbol]) provide a
+detailed view with more information prepended or appended to
+completions."
   :type 'boolean
   :version "28.1")