target_link_libraries: Document that new sigs privatize old (#13876)

Explain in the documentation for the legacy signature

 target_link_libraries(foo bar)

that the other signatures like

 target_link_libraries(foo LINK_INTERFACE_LIBRARIES ...)
 target_link_libraries(foo LINK_PRIVATE ...)

will set the LINK_INTERFACE_LIBRARIES target property and therefore make
libraries specified only by the legacy signature private.
Merge topic 'doc-link-interface' into next

f032fb9 target_link_libraries: Document that new sigs privatize old (#13876)