Comments following read through of WD11

Line 311 - UTF-8 is a character encoding, not a string encoding. Change "character-strings" to "characters".

Line 314 - Change "UTF-8 encoded strings" to "strings of UTF-8 encoded characters".

Line 687 - There is an extraneous comma: "payload.,Password"

Line 1346 - Including a reference to say "as defined in section 2.1.2" would be useful here, as this is specific to the mechanism by which MQTT stores the number of bytes, rather than UTF-8 itself.