Help: Mention INTERFACE_SOURCES as settable for INTERFACE libs

The add_library(INTERFACE) and Interface Libraries documentation
list all the INTERFACE_* properties and target_* commands that
can be used to define the interface.  Add INTERFACE_SOURCES
and target_sources() to these lists for completeness.

Help/command/add_library.rst

@@ -124,8 +124,9 @@ imported. Typically the ``INTERFACE_*`` properties are populated on
 the interface target using the :command:`set_property`,
 :command:`target_link_libraries(INTERFACE)`,
 :command:`target_include_directories(INTERFACE)`,
-:command:`target_compile_options(INTERFACE)`
-and :command:`target_compile_definitions(INTERFACE)` commands, and then it
+:command:`target_compile_options(INTERFACE)`,
+:command:`target_compile_definitions(INTERFACE)`,
+and :command:`target_sources(INTERFACE)` commands, and then it
 is used as an argument to :command:`target_link_libraries` like any other
 target.
 

Help/manual/cmake-buildsystem.7.rst

@@ -786,11 +786,12 @@ It may specify usage requirements such as
 :prop_tgt:`INTERFACE_COMPILE_DEFINITIONS`,
 :prop_tgt:`INTERFACE_COMPILE_OPTIONS`,
 :prop_tgt:`INTERFACE_LINK_LIBRARIES`, and
+:prop_tgt:`INTERFACE_SOURCES`,
 :prop_tgt:`INTERFACE_POSITION_INDEPENDENT_CODE`.
 Only the ``INTERFACE`` modes of the :command:`target_include_directories`,
 :command:`target_compile_definitions`, :command:`target_compile_options`,
-and :command:`target_link_libraries` commands may be used with ``INTERFACE``
-libraries.
+:command:`target_sources`, and :command:`target_link_libraries` commands
+may be used with ``INTERFACE`` libraries.
 
 A primary use-case for ``INTERFACE`` libraries is header-only libraries.