Document DDL queries

[mmnosek]

According to 2.3.1 release note, Starcounter supports some DDLs: "Partial DDL queries support. The ALTER/DROP TABLE and ADD/DROP COLUMN are now supported.". I cannot find anything about it in docs, it should be documented.

[k-rus]

Link to the internal documentation, which currently need to be copied manually into public documentation: https://starcounter.gitbooks.io/companytrack/content/NewQP%20SQL%20documentation/sql_ALTER_TABLE.html