Document custom command behavior without DEPENDS (#11407)

The behavior of add_custom_command when no DEPENDS option is specified
matches that of standard Make behavior, but it does not hurt to describe
it explicitly.

Source/cmAddCustomCommandCommand.h

@@ -152,6 +152,9 @@ public:
       "If any dependency is an OUTPUT of another custom command in the "
       "same directory (CMakeLists.txt file) CMake automatically brings the "
       "other custom command into the target in which this command is built.  "
+      "If DEPENDS is not specified the command will run whenever the OUTPUT "
+      "is missing; if the command does not actually create the OUTPUT then "
+      "the rule will always run.  "
       "If DEPENDS specifies any target (created by an ADD_* command) "
       "a target-level dependency is created to make sure the target is "
       "built before any target using this custom command.  Additionally, "