phpdoc in Table::addForeignKey() update

Added significant typehint in PhpDoc: `$foreignTable` param in `addForeignKeyConstraint()` can be just string name of table.

It unobvious in IDE hinting and treated as wrong param. It even can confuse developer, forcing him use foreign keys additions only using existing tables. That was my case, for example :confused: Lost time till found out, that Table schema not necessary to exist for Foreign Key referring .