Help: Document installed file property API

Since commit v3.1.0-rc1~479^2~1 (Add an "installed file" property
scope, 2014-05-15) the get_property and set_property commands
support an 'INSTALL' scope.  Add documentation for this scope.

Help/command/get_property.rst

@@ -10,6 +10,7 @@ Get a property.
                 DIRECTORY [dir]    |
                 TARGET    <target> |
                 SOURCE    <source> |
+                INSTALL   <file>   |
                 TEST      <test>   |
                 CACHE     <entry>  |
                 VARIABLE>
@@ -35,6 +36,9 @@ be one of the following:
 ``SOURCE``
   Scope must name one source file.
 
+``INSTALL``
+  Scope must name one installed file path.
+
 ``TEST``
   Scope must name one existing test.