Add / document a more convenient way to specify composite primary keys

[arogachev]

The way that was advised by @darkdef

$qb->createTable(
    'test',
    [
        'name' => 'varchar(255) NOT NULL',
        'email' => 'varchar(255) NOT NULL',
        'PRIMARY KEY (name, email)',
    ],
);

PRIMARY KEY (name, email):

• This is not a column.
• It's pretty basic for writing raw SQL.

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[arogachev]

Cycle implementation just for comparison:

$schema = $table->getSchema();
$schema->string('parent', 128)->nullable(false);
$schema->string('child', 128)->nullable(false);
$schema->setPrimaryKeys(['parent', 'child']);
$schema->save();

[samdark]

In Yii2 we decided that intentionally so when reviewing a database these names would make sense.

Automatic names when generated in a semantical way could:

1. Reach character limit. That is not uncommon for MySQL.
2. Collide.

That's why Doctrine when generating migrations via diff uses names such as UNIQ_56AB981964D218E or IDX_5A7E7FD6419054B4. That solves the issue technically but it becomes an unreadable mess to the one who will inspect the database itself later.