app_mp3: Document and warn about HTTPS incompatibility.

mpg123 doesn't support HTTPS, but the MP3Player application
doesn't document this or warn the user about this. HTTPS
streams have become more common nowadays and users could
reasonably try to play them without being aware they should
use the HTTP stream instead.

This adds documentation to note this limitation. It also
throws a warning if users try to use the HTTPS stream to
tell them to use the HTTP stream instead.

ASTERISK-29900 #close

Change-Id: Ie3b029be5258c5a701f71ed3b1a7a80d1e03b827