docs: add suggestions from API review to keyCopy() documentation
docs: add suggestions from API review to keyCopy() documentation

Added:
- Precondition
- Invariant
- Postcondition
- @see
- @since

Changed:
- moved Basic Usage Snippet in front of other examples
- Use @retval for for list of possible errors

ElektraInitiative/libelektra,mpranj/libelektra,petermax2/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,mpranj/libelektra,mpranj/libelektra,mpranj/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,petermax2/libelektra,mpranj/libelektra,mpranj/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,petermax2/libelektra,mpranj/libelektra,petermax2/libelektra,ElektraInitiative/libelektra,petermax2/libelektra