Document behavior of multiple target_link_libraries calls (#13113)

State explicitly that multiple calls append (rather than replace).

Source/cmTargetLinkLibrariesCommand.h

@@ -64,7 +64,8 @@ public:
       "Specify libraries or flags to use when linking a given target.  "
       "The named <target> must have been created in the current directory "
       "by a command such as add_executable or add_library.  "
-      "The remaining arguments specify library names or flags."
+      "The remaining arguments specify library names or flags.  "
+      "Repeated calls for the same <target> append items in the order called."
       "\n"
       "If a library name matches that of another target in the project "
       "a dependency will automatically be added in the build system to make "