Improve documentation for ECInstance and update ECClassId documentation that references it always being converted to classname

[ColinKerr]

This section does not give enough information on ECInstanceId and is out of date with ECClassId: https://www.itwinjs.org/learning/ecsql/#ecinstanceid-and-ecclassid

Consider adding additional information about ECInstanceId in glossary or other more general location than in ECSql docs