Help: Mention IMPORTED_LIBNAME_* in interface library property whitelist

This property was added by commit 09cda9d5 (Allow imported INTERFACE
libraries to specify a link library name, 2016-11-03) and is in the
implementation's whitelist.  Add it to the documentation too.

Help/manual/cmake-buildsystem.7.rst

@@ -964,6 +964,7 @@ are:
 * ``EXPORT_NAME``
 * ``IMPORTED``
 * ``NAME``
+* Properties matching ``IMPORTED_LIBNAME_*``
 * Properties matching ``MAP_IMPORTED_CONFIG_*``
 
 ``INTERFACE`` libraries may be installed and exported.  Any content they refer