Add clarifications and fix typographical errors

• Hex integers are → Hexadecimal representation is

• Specification encourages use of extended ISO-8601 format, but the first example is in basic format; I propose adding an example with a full date-time in extended format as the first example, which will be the one that gets copied&pasted by lazy people.

• predicates in `class expressions`: further filters for data in the subset (could be transformed into predicates for the `WHERE` clause): I’m not sure you can express an archetype predicate in WHERE

Second set of changes committed on this PR https://github.com/openEHR/specifications-QUERY/pull/5

Did the review of the current AQL v1.0.0 spec and reported issues here: https://docs.google.com/document/d/1g8zOh06LhSNi1yFZWKuBzUX0bJN88r7mKpAFqDNi2JI/edit?usp=sharing

Thomas reviewed the document and added comments / priorities.

I made some fixes based on Thomas' feedback, this is the working branch for those: https://github.com/openEHR/specifications-QUERY/tree/text_fixes

I’ll add some more fixes to the text before sending the first PR to review (to avoid sending too much changes on the same PR)