$queryCategoryOne = $mysql->query("SELECT * FROM rules WHERE category = 1");
$rulesCategoryOne = $queryCategoryOne->fetch_assoc();
<?php foreach($rulesCategoryOne as $rule): ?>
<span class="rules-intro__text">
<?=$rule['content']?>
</span>
<?php endforeach; ?>
<span class="rules-intro__text">
<?=$rulesCategoryOne['content']?>
</span>
$rulesCategoryAll = $queryCategoryOne->fetch_all();
// Я не знаю как у вас называется такой метод!!!
<?php foreach($rulesCategoryAll as $rule): ?>
<span class="rules-intro__text">
<?=$rule['content']?>
</span>
<?php endforeach; ?>