Clarify that Vector2.angle_to returns a signed angle

[3DartBlade]

Added a minor detail to the description of "angle_to" so that it specifies that the returned angle is signed. Hopefully helps others not to waste several hours not realizing this.

[tetrapod00]

For reference, the Vector3 equivalent functions differently and already clarifies the signedness:

godot/doc/classes/Vector3.xml

Lines 58 to 64 in 1a0bf54

	<method name="angle_to" qualifiers="const">
	<return type="float" />
	<param index="0" name="to" type="Vector3" />
	<description>
	Returns the unsigned minimum angle to the given vector, in radians.
	</description>
	</method>

[Repiteo]

Thanks! Congratulations on your first merged contribution! 🎉