Updated the intro paragragh based on the ICD-11 (the international equivalent of the US's DSM) for the following reasons:
* The previous text referred only to ADHD in children
* The previous text used the word 'disinterested' when I suspect the intended meaning was 'uninterested' but the sentence strucutre also needed a little tweaking
* The previous text defined attention based on what was interesting or not but that's not quite accurate
* The previous text only summarised focus (attention) but didn't summarise the other components of ADHD which are impulsivity and hyperactivity.
Added reading link to the ICD (for international readers where the DSM is not used)
