Add documentation and minor changes

[hannahbast]

1. Add documentation to sparql-hint.js and sparql.js (mostly the former) to better understand what is going on.

2. In context-insensitive mode, make the object AC queries half-sensitive, that is, take the predicate into account

3. Improve suggestions for variable names, e.g., a predicate hasAuthor now leads to the suggestion of an object variable ?author