Help: Format set_property and get_property command docs
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: Add 3.1 release notes for INSTALL property scope
Merge topic 'doc-installed-file-property' into next

7a4e3359 Help: Add 3.1 release notes for INSTALL property scope
0a8b0e69 Help: Document installed file property API
d74ea44c Help: Format set_property and get_property command docs