[docs] Improvement suggestion for docs of "plistlib"

[Bryan Dunphy]

In the segment on the Data class, add a suggestion to use “Latin-1” encoding if they need to store TRUE binary data (such as a picture or sound clip) since it has 256 characters that correspond directly to the 256 possible values of a byte. Meaning that if the string is written out to a file in BINARY mode it will EXACTLY recreate the original file.