doc: Amend comment about ZeroMQ config files

CMakeLists.txt

@@ -145,7 +145,9 @@ if(WITH_ZMQ)
     find_package(ZeroMQ CONFIG REQUIRED)
   else()
     # The ZeroMQ project has provided config files since v4.2.2.
-    # TODO: Switch to find_package(ZeroMQ) at some point in the future.
+    # However, mainstream distributions do not yet provide CMake
+    # config files for ZeroMQ packages. If they do in the future,
+    # find_package(ZeroMQ) may be used instead.
     find_package(PkgConfig REQUIRED)
     pkg_check_modules(libzmq REQUIRED IMPORTED_TARGET libzmq>=4)
   endif()