Corrected documentation for Material.isSolid()
The API documentation for Material.isSolid() is deceptive. It reflects behaviour from Minecraft's Material.isSolid() which seemingly has a different meaning.

The current API documentation for Material.isSolid() is "Check if the material is a block and solid (cannot be passed through by a player)" which is technically incorrect. For example, players can pass through SIGN_POST and WALL_SIGN without trouble. On the other hand, FLOWER_POT is treated as a non-solid whilst players can not traverse through it.

PerMaterialTest tests the Material.isSolid() the call for each material. It ensures Material.isSolid() matches its native counterpart for each material. However, the native counterpart more accurately defines whether or not a liquid breaks this item. For example: SIGN_POST does not break when water flows on it and is thus a solid block. However, FLOWER_POT does break when water flows upon it and is thus a non-solid block.

This commit changes the Material.isSolid() documentation to more accurately describe its behaviour.

Fixes BUKKIT-5729, BUKKIT-5671, BUKKIT-5559