Added documentation for some base Block classes

src/block/Flowable.php

@@ -26,6 +26,10 @@ namespace pocketmine\block;
 use pocketmine\block\utils\SupportType;
 use pocketmine\math\AxisAlignedBB;
 
+/**
+ * "Flowable" blocks are destroyed if water flows into the same space as the block. These blocks usually don't have any
+ * collision boxes, and can't provide support for other blocks.
+ */
 abstract class Flowable extends Transparent{
 
 	public function canBeFlowedInto() : bool{

src/block/Opaque.php

@@ -23,6 +23,10 @@ declare(strict_types=1);
 
 namespace pocketmine\block;
 
+/**
+ * Opaque blocks do not allow light to pass through. They are usually collidable full-cube blocks.
+ * Most blocks in Minecraft fall into this category.
+ */
 class Opaque extends Block{
 
 	public function isSolid() : bool{

src/block/Thin.php

@@ -29,6 +29,9 @@ use pocketmine\math\AxisAlignedBB;
 use pocketmine\math\Facing;
 use function count;
 
+/**
+ * Thin blocks behave like glass panes. They connect to full-cube blocks horizontally adjacent to them if possible.
+ */
 class Thin extends Transparent{
 	/** @var bool[] facing => dummy */
 	protected array $connections = [];

src/block/Transparent.php

@@ -23,6 +23,12 @@ declare(strict_types=1);
 
 namespace pocketmine\block;
 
+/**
+ * Transparent blocks do not block any light from propagating through them.
+ *
+ * Note: This does **not** imply that the block is **visually** transparent. For example, chests allow light to pass
+ * through, but the player cannot see through them except at the edges.
+ */
 class Transparent extends Block{
 
 	public function isTransparent() : bool{

src/block/UnknownBlock.php

@@ -25,6 +25,9 @@ namespace pocketmine\block;
 
 use pocketmine\item\Item;
 
+/**
+ * Represents a block which is unrecognized or not implemented.
+ */
 class UnknownBlock extends Transparent{
 
 	public function __construct(BlockIdentifier $idInfo, BlockBreakInfo $breakInfo){