Reference get_property() from old get_*_property() commands

The former is a much more general and modern command.  Refer authors to
it from documentation of its predecessors.

Source/cmGetCMakePropertyCommand.h

@@ -59,7 +59,9 @@ public:
         "If the property is not found, VAR will be set to \"NOTFOUND\".  "
         "Some supported properties "
         "include: VARIABLES, CACHE_VARIABLES, COMMANDS, MACROS, and "
-        "COMPONENTS.";
+        "COMPONENTS."
+        "\n"
+        "See also the more general get_property() command.";
     }
   
   cmTypeMacro(cmGetCMakePropertyCommand, cmCommand);

Source/cmGetDirectoryPropertyCommand.h

@@ -66,7 +66,8 @@ public:
         "Get a variable definition from a directory.  "
         "This form is useful to get a variable definition from another "
         "directory."
-        ;
+        "\n"
+        "See also the more general get_property() command.";
     }
   
   cmTypeMacro(cmGetDirectoryPropertyCommand, cmCommand);

Source/cmGetSourceFilePropertyCommand.h

@@ -53,7 +53,9 @@ public:
         "stored in the variable VAR.  If the property is not found, VAR "
         "will be set to \"NOTFOUND\". Use set_source_files_properties to set "
         "property values.  Source file properties usually control how the "
-        "file is built. One property that is always there is LOCATION";
+        "file is built. One property that is always there is LOCATION"
+        "\n"
+        "See also the more general get_property() command.";
     }
   
   cmTypeMacro(cmGetSourceFilePropertyCommand, cmCommand);

Source/cmGetTargetPropertyCommand.h

@@ -55,7 +55,9 @@ public:
         "property values.  Properties are usually used to control how "
         "a target is built, but some query the target instead.  "
         "This command can get properties for any target so far created. "
-        "The targets do not need to be in the current CMakeLists.txt file.";
+        "The targets do not need to be in the current CMakeLists.txt file."
+        "\n"
+        "See also the more general get_property() command.";
     }
   
   cmTypeMacro(cmGetTargetPropertyCommand, cmCommand);

Source/cmGetTestPropertyCommand.h

@@ -52,7 +52,9 @@ public:
       "Get a property from the Test.  The value of the property is "
       "stored in the variable VAR.  If the property is not found, VAR "
       "will be set to \"NOTFOUND\". For a list of standard properties "
-      "you can type cmake --help-property-list";
+      "you can type cmake --help-property-list"
+      "\n"
+      "See also the more general get_property() command.";
     }
 
   cmTypeMacro(cmGetTestPropertyCommand, cmCommand);