Document GPUParticles.emit_particle() only working on Forward+ and Mobile

Author: Calinou

doc/classes/GPUParticles2D.xml

@@ -38,6 +38,7 @@
 			<description>
 				Emits a single particle. Whether [param xform], [param velocity], [param color] and [param custom] are applied depends on the value of [param flags]. See [enum EmitFlags].
 				The default ParticleProcessMaterial will overwrite [param color] and use the contents of [param custom] as [code](rotation, age, animation, lifetime)[/code].
+				[b]Note:[/b] [method emit_particle] is only supported on the Forward+ and Mobile rendering methods, not Compatibility.
 			</description>
 		</method>
 		<method name="restart">

doc/classes/GPUParticles3D.xml

@@ -36,6 +36,7 @@
 			<description>
 				Emits a single particle. Whether [param xform], [param velocity], [param color] and [param custom] are applied depends on the value of [param flags]. See [enum EmitFlags].
 				The default ParticleProcessMaterial will overwrite [param color] and use the contents of [param custom] as [code](rotation, age, animation, lifetime)[/code].
+				[b]Note:[/b] [method emit_particle] is only supported on the Forward+ and Mobile rendering methods, not Compatibility.
 			</description>
 		</method>
 		<method name="get_draw_pass_mesh" qualifiers="const">