Issue #15566: Document encoding and errors parameters of TarInfo.frombuf().

Patch by Andy Holst.
diff --git a/Doc/library/tarfile.rst b/Doc/library/tarfile.rst
index 67796eb..4ca131b 100644
--- a/Doc/library/tarfile.rst
+++ b/Doc/library/tarfile.rst
@@ -504,7 +504,7 @@
    Create a :class:`TarInfo` object.
 
 
-.. method:: TarInfo.frombuf(buf)
+.. method:: TarInfo.frombuf(buf, encoding, errors)
 
    Create and return a :class:`TarInfo` object from string buffer *buf*.