ipv6.md: mention IPv4 mapped addresses

Reported-by: Josh Soref
Assisted-by: Jay Satiro
Fixes #13112
Closes #13131

docs/cmdline-opts/ipv6.md

@@ -22,3 +22,7 @@ Example:
 
 Use IPv6 addresses only when resolving hostnames, and not for example try
 IPv4.
+
+Your resolver may respond to an IPv6-only resolve request by returning IPv6
+addresses that contain "mapped" IPv4 addresses for compatibility purposes.
+macOS is known to do this.

tests/test1275.pl

@@ -31,6 +31,7 @@
 
 my %accepted=('curl' => 1,
               'libcurl' => 1,
+              'macOS' => 1,
               'c-ares' => 1);
 
 sub checkfile {
@@ -65,7 +66,7 @@ sub checkfile {
             $ignore ^= 1;
         }
         if(!$ignore) {
-            if(($prevl =~ /\.\z/) && ($line =~ /^( *)([a-z-]+)/)) {
+            if(($prevl =~ /\.\z/) && ($line =~ /^( *)([a-z][A-Za-z-]*)/)) {
                 my ($prefix, $word) = ($1, $2);
                 if($word =~ /^[a-z]/ && !$accepted{$word}) {
                     my $c = length($prefix);