Update Math.xml

Per the explanation for each quadrant and boundary, the returned value cannot equal to -π in any case. (it's -π < θ , not  -π  <= θ , in quadrant #2 )

Author: wangyoutian

xml/System/Math.xml

@@ -1021,7 +1021,7 @@
         <param name="y">The y coordinate of a point.</param>
         <param name="x">The x coordinate of a point.</param>
         <summary>Returns the angle whose tangent is the quotient of two specified numbers.</summary>
-        <returns>An angle, θ, measured in radians, such that -π ≤ θ ≤ π, and tan(θ) = <paramref name="y" /> / <paramref name="x" />, where (<paramref name="x" />, <paramref name="y" />) is a point in the Cartesian plane. Observe the following:
+        <returns>An angle, θ, measured in radians, such that -π < θ ≤ π, and tan(θ) = <paramref name="y" /> / <paramref name="x" />, where (<paramref name="x" />, <paramref name="y" />) is a point in the Cartesian plane. Observe the following:
 
 -   For (<paramref name="x" />, <paramref name="y" />) in quadrant 1, 0 &lt; θ &lt; π/2.