warn on man page that unencoded subject are not portable

[積丹尼 Dan Jacobson]

$ man mail

   -s, --subject=SUBJ
              send a message with the given SUBJECT

But warn that will be unreadable in Gmail('s Android app etc.), as the
subject will not be properly encoded.

$ echo test|mail -s 'ABC 測試' someuser@gmail.com

What will work needs to look like

$ echo test|mail -s 'ABC =?UTF-8?B?...' someuser@gmail.com

else the user will see 'ABC ???????' in Gmail.