api-extensions: add entry for idmapped_mounts

doc/api-extensions.md

@@ -135,3 +135,7 @@ Retrieve the seccomp notifier fd from a running container.
 ## seccomp\_proxy\_send\_notify\_fd
 
 Whether the seccomp notify proxy sends a long a notify fd file descriptor.
+
+## idmapped\_mounts
+
+Whether this LXC instance can handle idmapped mounts.

src/lxc/api_extensions.h

@@ -41,6 +41,7 @@ static char *api_extensions[] = {
 	"devpts_fd",
 	"seccomp_notify_fd_active",
 	"seccomp_proxy_send_notify_fd",
+	"idmapped_mounts",
 };
 
 static size_t nr_api_extensions = sizeof(api_extensions) / sizeof(*api_extensions);