Docs: improvements for better rendering in PHPDocumentor

[jrfnl]

• Remove markdown from DocBlock summaries as it's not supported.
• Add backticks around inline code elements in long descriptions and tag descriptions.
• Add markdown where relevant in long descriptions and tag descriptions to improve readability of the rendered output.
• Add inline {@see ...} tags to create links to other documented elements.
• Add inline {@link ...} tags.
• Replace <code> tags with code block syntax.
• Lower the amount of indentation used in code blocks for more readable rendering.
• Replace singular backticks for multi-line code blocks with triple backticks, to allow for rendering as a proper code block instead of inline.
• Replace some semi-lists with list syntax supported via markdown for better rendering.
• Replace some list like return value array definitions with code blocks for better rendering.
• Make some "Note:" remarks use blockquote syntax to draw more attention to them.
• Use uppercase true/false/null in @return tags to make them more easily readable at a glance.