Clarify the status of bundled external perl modules

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9826)

external/perl/Downloaded.txt

@@ -3,8 +3,9 @@ Intro
 
 If we find a useful Perl module that isn't one of the core Perl
 modules, we may choose to bundle it with the OpenSSL source.
+They remain unmodified and retain their copyright and license.
 
-Here, we simply list those modules and where we downloaded them from.
+Here, we simply list those modules and where we got them from.
 
 Downloaded and bundled Perl modules
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