Clarify the difference between uniforms and per-instance uniforms in docs

doc/classes/GeometryInstance3D.xml

@@ -21,7 +21,10 @@
 			<param index="0" name="name" type="StringName" />
 			<param index="1" name="value" type="Variant" />
 			<description>
-				Set the value of a shader parameter for this instance only.
+				Set the value of a shader uniform for this instance only ([url=$DOCS_URL/tutorials/shaders/shader_reference/shading_language.html#per-instance-uniforms]per-instance uniform[/url]). See also [method ShaderMaterial.set_shader_parameter] to assign a uniform on all instances using the same [ShaderMaterial].
+				[b]Note:[/b] For a shader uniform to be assignable on a per-instance basis, it [i]must[/i] be defined with [code]instance uniform ...[/code] rather than [code]uniform ...[/code] in the shader code.
+				[b]Note:[/b] [param name] is case-sensitive and must match the name of the uniform in the code exactly (not the capitalized name in the inspector).
+				[b]Note:[/b] Per-instance shader uniforms are currently only available in 3D, so there is no 2D equivalent of this method.
 			</description>
 		</method>
 	</methods>

doc/classes/ShaderMaterial.xml

@@ -23,7 +23,8 @@
 			<param index="1" name="value" type="Variant" />
 			<description>
 				Changes the value set for this material of a uniform in the shader.
-				[b]Note:[/b] [param param] must match the name of the uniform in the code exactly.
+				[b]Note:[/b] [param param] is case-sensitive and must match the name of the uniform in the code exactly (not the capitalized name in the inspector).
+				[b]Note:[/b] Changes to the shader uniform will be effective on all instances using this [ShaderMaterial]. To prevent this, use per-instance uniforms with [method GeometryInstance3D.set_instance_shader_parameter] or duplicate the [ShaderMaterial] resource using [method Resource.duplicate]. Per-instance uniforms allow for better shader reuse and are therefore faster, so they should be preferred over duplicating the [ShaderMaterial] when possible.
 			</description>
 		</method>
 	</methods>