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.
 

Help/command/set_property.rst

@@ -9,6 +9,7 @@ Set a named property in a given scope.
                 DIRECTORY [dir]                   |
                 TARGET    [target1 [target2 ...]] |
                 SOURCE    [src1 [src2 ...]]       |
+                INSTALL   [file1 [file2 ...]]     |
                 TEST      [test1 [test2 ...]]     |
                 CACHE     [entry1 [entry2 ...]]>
                [APPEND] [APPEND_STRING]
@@ -34,6 +35,22 @@ be one of the following:
   file properties are visible only to targets added in the same
   directory (CMakeLists.txt).
 
+``INSTALL``
+  Scope may name zero or more installed file paths.
+  These are made available to CPack to influence deployment.
+
+  Both the property key and value may use generator expressions.
+  Specific properties may apply to installed files and/or directories.
+
+  Path components have to be separated by forward slashes,
+  must be normalized and are case sensitive.
+
+  To reference the installation prefix itself with a relative path use ".".
+
+  Currently installed file properties are only defined for
+  the WIX generator where the given paths are relative
+  to the installation prefix.
+
 ``TEST``
   Scope may name zero or more existing tests.