add POSIX disclaimers

Author: Anton-Latukha

scripts/install-nix-from-closure.sh

@@ -10,7 +10,7 @@
 ###
 ### Upstream URL: https://github.com/NixOS/nix/blob/master/scripts/install-nix-from-closure.sh
 ###
-### Script is fully POSIX compatible, with controlled exceptions.
+### Script is fully POSIX compatible (for standard see http://pubs.opengroup.org/onlinepubs/9699919799/), with controlled exceptions.
 ### (must be) Fully transactional. That means, if it not succeeded - it does rolls back.
 ###
 ### `/bin/sh -u` is not possible to do, because Docker environments has USER='', and shell checks that and not get even to execution part
@@ -33,7 +33,9 @@
 
 # Special things about this script
 #
-# 1) Notice, Warning, Error, ErrorRevert massages have special form to be very readable and get attention.
+# 1) Since this script tries to be fully POSIX compatible, code is influenced by that. There are day-to-day commands and solutions we get used to, but they are not in POSIX standard. So if you try to simplify some operation, please check it with http://pubs.opengroup.org/onlinepubs/9699919799/ .
+#
+# 2) Notice, Warning, Error, ErrorRevert massages have special form to be very readable and get attention.
 # Message body starts from new line.
 # And has 4 spaces from the left. Always.
 #