docs(chflags): add references

Author: akinomyoga

completions/chflags

@@ -2,6 +2,12 @@
 
 [[ $OSTYPE == *@(bsd|darwin)* ]] || return 1
 
+# References
+#
+# [1] FreeBSD - https://man.freebsd.org/cgi/man.cgi?chflags(1)
+# [2] NetBSD - https://man.netbsd.org/NetBSD-9.0/chflags.1
+# [3] OpenBSD - https://man.openbsd.org/chflags.1
+
 _comp_cmd_chflags()
 {
     local cur prev words cword comp_args