Add help text for `fastboot gsi status`

Bug: 298533290
Test: fastboot -h
Change-Id: Icf91a980059aaa8e74e41af4036778fbbfbf1bd2
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 235d723..12a1ddc 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -571,7 +571,8 @@
             "                            Format a flash partition.\n"
             " set_active SLOT            Set the active slot.\n"
             " oem [COMMAND...]           Execute OEM-specific command.\n"
-            " gsi wipe|disable           Wipe or disable a GSI installation (fastbootd only).\n"
+            " gsi wipe|disable|status    Wipe, disable or show status of a GSI installation\n"
+            "                            (fastbootd only).\n"
             " wipe-super [SUPER_EMPTY]   Wipe the super partition. This will reset it to\n"
             "                            contain an empty set of default dynamic partitions.\n"
             " create-logical-partition NAME SIZE\n"