Help: Simplify CMAKE_ENABLE_EXPORTS documentation

In real projects the variable form should rarely be used because the
decision to export symbols from an executable is very specific.
Remove its main description, which duplicates the `ENABLE_EXPORTS`
target property, and simply reference the property instead.

Help/variable/CMAKE_ENABLE_EXPORTS.rst

@@ -1,22 +1,8 @@
 CMAKE_ENABLE_EXPORTS
 --------------------
 
-Specify whether an executable exports symbols for loadable modules.
+Specify whether executables export symbols for loadable modules.
 
-Normally an executable does not export any symbols because it is the
-final program.  It is possible for an executable to export symbols to
-be used by loadable modules.  When this property is set to true CMake
-will allow other targets to ``link`` to the executable with the
-:command:`TARGET_LINK_LIBRARIES` command.  On all platforms a target-level
-dependency on the executable is created for targets that link to it.
-For DLL platforms an import library will be created for the exported
-symbols and then used for linking.  All Windows-based systems
-including Cygwin are DLL platforms.  For non-DLL platforms that
-require all symbols to be resolved at link time, such as macOS, the
-module will ``link`` to the executable using a flag like
-``-bundle_loader``.  For other non-DLL platforms the link rule is simply
-ignored since the dynamic loader will automatically bind symbols when
-the module is loaded.
-
-This variable is used to initialize the target property
-:prop_tgt:`ENABLE_EXPORTS` for executable targets.
+This variable is used to initialize the :prop_tgt:`ENABLE_EXPORTS` target
+property for executable targets when they are created by calls to the
+:command:`add_executable` command.  See the property documentation for details.