A......... [ZBX-8634] Fix code style and other minor improvements. 
- Replace !is_null() to "!== null" and is_null() to "=== null".
- Rename variables in methods so to $triggerPrototype where appropriate.
- Add missing and update existing phpDocs as discussed.
- Add @see links for public methods.
- Fix comments so they do not lie.
- In CTriggerPrototype::delete() simplify code deleting triggers created from trigger prototype.