nghttpx: Add explanation about units in <SIZE>

src/shrpx.cc

@@ -1184,7 +1184,10 @@ Misc:
   --conf=<PATH>      Load configuration from <PATH>.
                      Default: )" << get_config()->conf_path.get() << R"(
   -v, --version      Print version and exit.
-  -h, --help         Print this help and exit.)" << std::endl;
+  -h, --help         Print this help and exit.
+
+  The <SIZE> argument is an integer and an optional unit (e.g., 10K is
+  10 * 1024).  Units are K, M and G (powers of 1024).)" << std::endl;
 }
 } // namespace