Help: Clarify policy `CMP0040` documentation (#15681)

State explicitly that the target must be defined in the current
directory.  While at it, improve markup formatting.

Help/policy/CMP0040.rst

 CMP0040
 -------
 
-The target in the TARGET signature of add_custom_command() must exist.
+The target in the ``TARGET`` signature of :command:`add_custom_command`
+must exist and must be defined in current directory.
 
 CMake 2.8.12 and lower silently ignored a custom command created with
-the TARGET signature of :command:`add_custom_command`
-if the target is unknown.
+the ``TARGET`` signature of :command:`add_custom_command`
+if the target is unknown or was defined outside the current directory.
 
-The OLD behavior for this policy is to ignore custom commands
-for unknown targets. The NEW behavior for this policy is to report an error
-if the target referenced in :command:`add_custom_command` is unknown.
+The ``OLD`` behavior for this policy is to ignore custom commands
+for unknown targets.  The ``NEW`` behavior for this policy is to report
+an error if the target referenced in :command:`add_custom_command` is
+unknown or was defined outside the current directory.
 
 This policy was introduced in CMake version 3.0.  CMake version
-|release| warns when the policy is not set and uses OLD behavior.  Use
-the cmake_policy command to set it to OLD or NEW explicitly.
+|release| warns when the policy is not set and uses ``OLD`` behavior.
+Use the :command:`cmake_policy` command to set it to ``OLD`` or
+``NEW`` explicitly.
 
 .. include:: DEPRECATED.txt