Document advantage of executables over executable (#7505)

* Document advantage of `executables` over `executable`

One aspect of #5852 revealed that this feature is under-documented, so
documenting the current behavior is a good first step.

doc/stanzas/executable.rst

@@ -8,9 +8,9 @@ executable stanzas is as follows:
 
 .. code:: dune
 
-    (executable
-     (name <name>)
-     <optional-fields>)
+   (executable
+    (name <name>)
+    <optional-fields>)
 
 ``<name>`` is a module name that contains the executable's main entry point.
 There can be additional modules in the current directory; you only need to
@@ -50,9 +50,9 @@ files for executables. See `executables_implicit_empty_intf`_.
 
    .. code:: dune
 
-       (install
-        (section bin)
-        (files (<name>.exe as <public-name>)))
+      (install
+       (section bin)
+       (files (<name>.exe as <public-name>)))
 
 .. _shared-exe-fields:
 
@@ -172,9 +172,9 @@ executables and native shared objects:
 
 .. code:: dune
 
-          (executables
-            (names a b c)
-            (modes (byte exe) (native shared_object)))
+   (executables
+     (names a b c)
+     (modes (byte exe) (native shared_object)))
 
 Additionally, you can use the following shorthands:
 
@@ -191,10 +191,10 @@ For instance, the following ``modes`` fields are all equivalent:
 
 .. code:: dune
 
-          (modes (exe object shared_object))
-          (modes ((best exe)
-                  (best object)
-                  (best shared_object)))
+   (modes (exe object shared_object))
+   (modes ((best exe)
+           (best object)
+           (best shared_object)))
 
 Lastly, use the special mode ``byte_complete`` for building a bytecode
 executable as a native self-contained executable, i.e., an executable that
@@ -270,10 +270,11 @@ executables
 
 There is a very subtle difference in the naming of these stanzas. One is
 ``executables``, plural, and the other is ``executable``, singular. The
-``executables`` stanza is the same as the ``executable`` stanza except that it's
-used to describe several executables sharing the same configuration, so the
+``executables`` stanza is very similar as the ``executable`` stanza but can be
+used to to describe several executables sharing the same configuration, so the
 plural ``executables`` stanza is used to describe more than one executable.
 
+
 It shares the same fields as the ``executable`` stanza, except that instead of
 ``(name ...)`` and ``(public_name ...)`` you must use the plural versions as
 well:
@@ -286,3 +287,32 @@ well:
   executable. The list of names must be of the same length as the list in the
   ``(names ...)`` field. Moreover, you can use ``-`` for executables that
   shouldn't be installed.
+
+However, using ``executables`` the executables defined in the stanza are
+allowed to share modules.
+
+Given modules ``Foo``, ``Bar`` and ``Baz`` the usage of ``executables`` can
+simplify the code:
+
+.. code:: dune
+
+   (executables
+     (names foo bar))
+
+Instead of the more complex
+
+.. code:: dune
+
+   (library
+     (name baz)
+     (modules baz))
+   
+   (executable
+     (name foo)
+     (modules foo)
+     (libraries baz))
+   
+   (executable
+     (name bar)
+     (modules bar)
+     (libraries baz))