Help: Update get_property and get_source_file_property docs

Specify the names and alternatives a bit more clearly.

Help/command/get_property.rst

@@ -10,7 +10,7 @@ Get a property.
                 DIRECTORY [<dir>]  |
                 TARGET    <target> |
                 SOURCE    <source> |
-                          [<TARGET_DIRECTORY ... | DIRECTORY ...>]   |
+                          [TARGET_DIRECTORY <target> | DIRECTORY <dir>] |
                 INSTALL   <file>   |
                 TEST      <test>   |
                 CACHE     <entry>  |

Help/command/get_source_file_property.rst

@@ -5,7 +5,7 @@ Get a property for a source file.
 
 .. code-block:: cmake
 
-  get_source_file_property(VAR file [<TARGET_DIRECTORY ... | DIRECTORY ...>] property)
+  get_source_file_property(VAR file [TARGET_DIRECTORY <target> | DIRECTORY <dir>] property)
 
 Gets a property from a source file.  The value of the property is
 stored in the variable ``VAR``.  If the source property is not found, the