Ruby

[DOC] Addition to encoding.rdoc (#5617)

Adds section "Transcoding a Stream," listing relevant methods in IO. Moves an example from section "String Encoding Example" to the new section. Removes header "String Encoding Example" for now-empty section. Changes items in section "Transcoding a String" from labeled list items to bullet list items. (Labeled list items are sometimes rendered with strange indentations for continued lines, and are always rendered with different indentations for the items.)