Document `Vector2.dot()` and `Vector3.dot()` more extensively

These methods are commonly used in games. It's time to make the
documentation more explicit about them :)

doc/classes/Vector2.xml

@@ -154,7 +154,10 @@
 			<argument index="0" name="with" type="Vector2">
 			</argument>
 			<description>
-				Returns the dot product with vector [code]b[/code].
+				Returns the dot product with vector [code]b[/code]. This can be used to compare the angle between two vectors. For example, this can be used to determine whether an enemy is facing the player.
+				The dot product will be [code]0[/code] for a straight angle (90 degrees), greater than 0 for angles narrower than 90 degrees and lower than 0 for angles wider than 90 degrees.
+				When using unit (normalized) vectors, the result will always be between [code]-1.0[/code] (180 degree angle) when the vectors are facing opposite directions, and [code]1.0[/code] (0 degree angle) when the vectors are aligned.
+				[b]Note:[/b] [code]a.dot(b)[/code] is equivalent to [code]b.dot(a)[/code].
 			</description>
 		</method>
 		<method name="floor">

doc/classes/Vector3.xml

@@ -123,7 +123,10 @@
 			<argument index="0" name="b" type="Vector3">
 			</argument>
 			<description>
-				Returns the dot product with [code]b[/code].
+				Returns the dot product with vector [code]b[/code]. This can be used to compare the angle between two vectors. For example, this can be used to determine whether an enemy is facing the player.
+				The dot product will be [code]0[/code] for a straight angle (90 degrees), greater than 0 for angles narrower than 90 degrees and lower than 0 for angles wider than 90 degrees.
+				When using unit (normalized) vectors, the result will always be between [code]-1.0[/code] (180 degree angle) when the vectors are facing opposite directions, and [code]1.0[/code] (0 degree angle) when the vectors are aligned.
+				[b]Note:[/b] [code]a.dot(b)[/code] is equivalent to [code]b.dot(a)[/code].
 			</description>
 		</method>
 		<method name="floor">