[CALCITE-3262] Refine doc of SubstitutionVisitor.java

[jinxing64]

Current doc of SubstitutionVisitor.java says the supported core relational operators are @link org.apache.calcite.rel.logical.LogicalTableScan, and so on.
But with convertTableAccess=true (https://github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/sql2rel/SqlToRelConverter.java#L5636), it's a EnumerableTableScan under MutableScan, which is inconsistent with the doc.
And what's more, MutableRels and SubstitutionVisitor supporting scope doesn't limit to be org.apache.calcite.rel.logical.LogicalXXX.
So I think it might make sense to update/refine the doc to say that the supported core relational operators are

 * {@link org.apache.calcite.rel.core.TableScan},
 * {@link org.apache.calcite.rel.core.Filter},
 * {@link org.apache.calcite.rel.core.Project},
 * {@link org.apache.calcite.rel.core.Join},
 * {@link org.apache.calcite.rel.core.Union},
 * {@link org.apache.calcite.rel.core.Aggregate}.</p>

[hsyuan]

+1

[jinxing64]

@hsyuan
Thanks a lot for approve !

[jinxing64]

Thanks for merging ~