Document some dex2oat command line options.

Also process option `--dump-cfg-append` correctly.

Test: m build-art-host
Change-Id: I062154c267c1a6eec22af42d985f19a555bdf4ca
diff --git a/compiler/driver/compiler_options.cc b/compiler/driver/compiler_options.cc
index a0c0a2a..a4e2083 100644
--- a/compiler/driver/compiler_options.cc
+++ b/compiler/driver/compiler_options.cc
@@ -200,7 +200,7 @@
     ParseDumpInitFailures(option, Usage);
   } else if (option.starts_with("--dump-cfg=")) {
     dump_cfg_file_name_ = option.substr(strlen("--dump-cfg=")).data();
-  } else if (option.starts_with("--dump-cfg-append")) {
+  } else if (option == "--dump-cfg-append") {
     dump_cfg_append_ = true;
   } else if (option.starts_with("--register-allocation-strategy=")) {
     ParseRegisterAllocationStrategy(option, Usage);