[doc] piconv: say in *some* implementations of iconv

[jidanni]

piconv man page says:

   -f,--from from_encoding
       Specifies the encoding you are converting from.  Unlike iconv, this option can be omitted.  In such
       cases, the current locale is used.

   -t,--to to_encoding
       Specifies the encoding you are converting to.  Unlike iconv, this option can be omitted.  In such
       cases, the current locale is used.

But in most / some / current ? real iconv it can also be omitted. So say "in some implementations of iconv...'

       Therefore, when both -f and -t are omitted, piconv just acts like cat.

Well it actually just prints a usage message.

[richardleach]

piconv is pulled in from Encode (https://metacpan.org/dist/Encode) which is a CPAN-first module.

It already has some open tickets regarding the behaviour when both -f and -t are omitted, e.g.

• https://rt.cpan.org/Ticket/Display.html?id=146804
• https://rt.cpan.org/Ticket/Display.html?id=146805

@jidanni - If you want to raise any tickets directly against Encode regarding piconv documentation of iconv behaviour, https://rt.cpan.org/Public/Dist/Display.html?Name=Encode or https://github.com/dankogai/p5-encode are the places to do it.