man page: Mention --follow-firmlinks

doc/ncdu.pod

@@ -179,11 +179,16 @@ this option will not follow symlinks to directories and will count each
 symlinked file as a unique file (i.e. unlike how hard links are handled). This
 is subject to change in later versions.
 
+=item --follow-firmlinks
+
+(MacOS only) follow firmlinks. WARNING: This may cause directories to be
+scanned and counted multiple times. Firmlink cycles are current (1.15) not
+detected, so it may also cause ncdu to get stuck in an infinite loop and
+eventually run out of memory.
+
 =item --exclude-kernfs
 
-Exclude Linux pseudo filesystems, e.g. /proc (procfs), /sys (sysfs).
-This option is only available on Linux, but exported statistics can be loaded on
-all platforms.
+(Linux only) Exclude Linux pseudo filesystems, e.g. /proc (procfs), /sys (sysfs).
 
 The complete list of currently known pseudo filesystems is: binfmt, bpf, cgroup,
 cgroup2, debug, devpts, proc, pstore, security, selinux, sys, trace.