Rework use_texture_alpha to provide three opaque/clip/blend modes

The change that turns nodeboxes and meshes opaque when possible is kept,
as is the compatibility code that warns modders to adjust their nodedefs.

doc/lua_api.txt

@@ -4386,6 +4386,8 @@ Utilities
           object_step_has_moveresult = true,
           -- Whether get_velocity() and add_velocity() can be used on players (5.4.0)
           direct_velocity_on_players = true,
+          -- nodedef's use_texture_alpha accepts new string modes (5.4.0)
+          use_texture_alpha_string_modes = true,
       }
 
 * `minetest.has_feature(arg)`: returns `boolean, missing_features`
@@ -7340,10 +7342,18 @@ Used by `minetest.register_node`.
         -- If the node has a palette, then this setting only has an effect in
         -- the inventory and on the wield item.
 
-        use_texture_alpha = false,
-        -- Use texture's alpha channel
-        -- If this is set to false, the node will be rendered fully opaque
-        -- regardless of any texture transparency.
+        use_texture_alpha = ...,
+        -- Specifies how the texture's alpha channel will be used for rendering.
+        -- possible values:
+        -- * "opaque": Node is rendered opaque regardless of alpha channel
+        -- * "clip": A given pixel is either fully see-through or opaque
+        --           depending on the alpha channel being below/above 50% in value
+        -- * "blend": The alpha channel specifies how transparent a given pixel
+        --            of the rendered node is
+        -- The default is "opaque" for drawtypes normal, liquid and flowingliquid;
+        -- "clip" otherwise.
+        -- If set to a boolean value (deprecated): true either sets it to blend
+        -- or clip, false sets it to clip or opaque mode depending on the drawtype.
 
         palette = "palette.png",
         -- The node's `param2` is used to select a pixel from the image.