Preset documentation for <check> elements

[Simon Poole]

TaggingPresets#Attributes says the following on the default values (explicitly set and implied):

default="default_value"

An explicitly declared default value for the item. This value is only applied through a preset when the object the preset is applied to has zero keys. If a value for default is not given, different implied default values apply depending upon the type:

• <check> (whether a checkbox is ticked on or off by default): implied default is "off"
• <text>, <combo>, <multiselect>: implied default is ""

However the actual JOSM behaviour seems to be to assume undefined / "" for <check> elements and not the default value for the "off" state ("no").