Use correct value for PECL DS MIN_CAPACITY constants

[LeonMelis]

9 years ago (back in the PHP 5.5 days), the PECL ds codebase was updated to use 8 as minimal size of various vector and hash table implementations as part of the PECL DS 1.2.0 release (released August 3rd, 2017). The PHP documentation was never updated though.

This PR updates the documentation values to all reflect the correct MIN_CAPACITY constant value for the various PECL DS classes. This PR also adds this to the changelog entries, for sake of completeness.

See:

• https://github.com/php-ds/ext-ds/releases/tag/v1.2.0

And a few of the specific commits:

• php-ds/ext-ds@9904fca
• php-ds/ext-ds@1e79457

The following code verifies that all default/minimal capacities in the various DS classes are indeed set to 8:

<?php

$classes = [
    \Ds\Vector::class,
    \Ds\Deque::class,
    \Ds\Map::class,
    \Ds\Set::class,
    \Ds\Stack::class,
    \Ds\Queue::class,
    \Ds\PriorityQueue::class,
];

foreach ($classes as $class) {
    $instance = new $class();
    $constant = defined("$class::MIN_CAPACITY") ? $class::MIN_CAPACITY : '<not defined>';
    echo "{$class}: constant={$constant} capacity={$instance->capacity()}\n";
}

Expected output (on any PECL version >=1.2.0 and/or PHP >=5.6):

Ds\Vector: constant=8 capacity=8
Ds\Deque: constant=8 capacity=8
Ds\Map: constant=8 capacity=8
Ds\Set: constant=8 capacity=8
Ds\Stack: constant=<not defined> capacity=8
Ds\Queue: constant=8 capacity=8
Ds\PriorityQueue: constant=8 capacity=8

I also checked the documentation for the other DS classes, those al seem to be correct.

[Girgias]

Thank you!