Update compilers.asciidoc
Update compilers.asciidoc

Updating documentation:
- fixed optional clause 
- updated optional clause limitation while in nesting with union & order-by
- went through other examples and limitations
robertdale/tinkerpop,pluradj/incubator-tinkerpop,pluradj/incubator-tinkerpop,robertdale/tinkerpop,krlohnes/tinkerpop,robertdale/tinkerpop,krlohnes/tinkerpop,pluradj/incubator-tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/tinkerpop,apache/incubator-tinkerpop,krlohnes/tinkerpop,apache/tinkerpop,robertdale/tinkerpop,robertdale/tinkerpop,apache/tinkerpop,krlohnes/tinkerpop,apache/tinkerpop,krlohnes/tinkerpop