Clarify the query support for secondary types

The query support for secondary types is not clear.

CMIS-742 defined the query behaviour for secondary types in CMIS 1.1.

CMIS-742 and section 2.1.14.2 of the 1.1 specification imply that secondary types can be used alone.

For example
SELECT * from cmis:rm_hold

There are two issues with this.

1) It implies that heterogeneous objects may be returned from a query, as the same secondary type could be applied to both documents and folders. Heterogeneous object can not be returned for all repositories.

2) Section 2.1.14.2.1, BNF Grammar, allows only <table name> in both the <from clause> and <joined table>. <table name> is defined to identify a primary type. There is a <secondary type table name> defined but only used in <column reference>. It looks like this was overlooked in the changes made for CMIS-742. The current BNF implies that only primary types may appear in FROM and JOIN clauses.