Doc updates explaining bug and possible workaround

Bug:  Alphabetise plugin: Single character titles not displayed in list
#3
The key field text, e.g. 'title' in the example, must be longer than
one character otherwise it will be ignored, thanks to Grant for
pointing this out. This only applies to the key field, so you can use a
separate 'sort' text field and have this use double characters and then
use this as the key.