Add column modifiers docs for PostgreSQL spatial column type

[aanfarhan]

It's possible to change spatial column type (PostgreSQL) between the default geography become geometry by adding column modifiers isGeometry().
In one of my project I was having trouble how to create database migration of spatial column type geometry, but fortunately solved my problem after reading this code Illuminate/Database/Schema/Grammars/PostgresGrammar.php#L943

Example of migrations code:

Schema::create('some_table', function (Blueprint $table) {
    //....
    $table->geometry('geometry_column')->nullable()->isGeometry(); // this generate spatial column type geometry
    $table->geometry('geography_column')->nullable(); // this generate spatial column type geography
    //....
});

By submitting this PR I hope that anyone can solve this same problem by reading the Laravel documentation. Thanks.