Change array type on ApiQueryBase::addTables

Change array type on ApiQueryBase::addTables

This function results in passing its value to IDatabase::select as first
parameter $table.
The first parameter is documented as string|array, because it can take
more complex arrays:
	 * Joins using parentheses for grouping (since MediaWiki 1.31) may be
	 * constructed using nested arrays. For example,
	 *
	 *    [ 'tableA', 'nestedB' => [ 'tableB', 'b2' => 'tableB2' ] ]

Update the type to allow also passing such complex arrays to this
function
ApiQueryBase::showHiddenUsersAddBlockInfo() is using this format

Change-Id: I8038243545bc1c79a1106ccc3bec7738e63c2e2e