Update MultiMeshInstance and WorldEnvironment documentation

With some edits by @akien-mga.

doc/classes/MultiMeshInstance.xml

@@ -4,7 +4,8 @@
 		Node that instances a [MultiMesh].
 	</brief_description>
 	<description>
-		MultiMeshInstance is a [Node] that takes a [MultiMesh] resource and adds it to the current scenario by creating an instance of it (yes, this is an instance of instances).
+		[code]MultiMeshInstance[/code] is a specialized node to instance [GeometryInstance]s based on a [MultiMesh] resource.
+		This is useful to optimize the rendering of a high amount of instances of a given mesh (for example tree in a forest or grass strands).
 	</description>
 	<tutorials>
 	</tutorials>
@@ -14,6 +15,7 @@
 	</methods>
 	<members>
 		<member name="multimesh" type="MultiMesh" setter="set_multimesh" getter="get_multimesh">
+			The [MultiMesh] resource that will be used and shared among all instances of the [code]MultiMeshInstance[/code].
 		</member>
 	</members>
 	<constants>

doc/classes/WorldEnvironment.xml

@@ -1,10 +1,12 @@
 <?xml version="1.0" encoding="UTF-8" ?>
 <class name="WorldEnvironment" inherits="Node" category="Core" version="3.0-rc1">
 	<brief_description>
-		Sets environment properties for the entire scene
+		Default environment properties for the entire scene (post-processing effects, lightning and background settings).
 	</brief_description>
 	<description>
-		The [code]WorldEnvironment[/code] node can be added to a scene in order to set default [Environment] variables for the scene. The [code]WorldEnvironment[/code] can be overridden by an [Environment] node set on the current [Camera]. Additionally, only one [code]WorldEnvironment[/code] may be instanced in a given scene at a time. The [code]WorldEnvironment[/code] allows the user to specify default lighting parameters (e.g. ambient lighting), various post-processing effects (e.g. SSAO, DOF, Tonemapping), and how to draw the background (e.g. solid color, skybox).
+		The [code]WorldEnvironment[/code] node is used to configure the default [Environment] for the scene.
+		The parameters defined in the [code]WorldEnvironment[/code] can be overridden by an [Environment] node set on the current [Camera]. Additionally, only one [code]WorldEnvironment[/code] may be instanced in a given scene at a time.
+		The [code]WorldEnvironment[/code] allows the user to specify default lighting parameters (e.g. ambient lighting), various post-processing effects (e.g. SSAO, DOF, Tonemapping), and how to draw the background (e.g. solid color, skybox). Usually, these are added in order to improve the realism/color balance of the scene.
 	</description>
 	<tutorials>
 		http://docs.godotengine.org/en/3.0/learning/features/3d/environment_and_post_processing.html
@@ -15,6 +17,7 @@
 	</methods>
 	<members>
 		<member name="environment" type="Environment" setter="set_environment" getter="get_environment">
+			The [Environment] resource used by this [code]WorldEnvironment[/code], defining the default properties.
 		</member>
 	</members>
 	<constants>