Add documentation for warn_early_init_failure()

Because the odds of someone altering the early init sequence in the
future and failing to realize the purpose of this code aren't low
enough. (I really should've included it in the main commit.)

src/game.cpp

@@ -414,6 +414,15 @@ static void init_locale() {
 	textdomain (PACKAGE);
 }
 
+/**
+ * Print an alert and instructions to stderr about early initialization errors.
+ *
+ * This is provided as an aid for users dealing with potential data dir
+ * configuration issues. The first code to read core WML *has* the
+ * responsibility to call this function in the event of a problem, to inform
+ * the user of the most likely possible cause and suggest a course of action
+ * to solve the issue.
+ */
 static void warn_early_init_failure()
 {
 	// NOTE: wrap output to 80 columns.
@@ -452,6 +461,7 @@ static int do_gameloop(int argc, char** argv)
 	res = font::load_font_config();
 	if(res == false) {
 		std::cerr << "could not initialize fonts\n";
+		// The most common symptom of a bogus data dir path -- warn the user.
 		warn_early_init_failure();
 		return 1;
 	}