atan2: adjust wording a bit, fix formatting

Swapping "the x-axis" and the description of the vector makes the
sentence a bit easier to understand, IMO. I think it's useful to
mention that unary `atan2` is identical to `atan`.

Also re-wrap atan2 section and remove trailing whitespace

Author: treyharris

doc/Type/Cool.pod6

@@ -288,22 +288,26 @@ Defined as:
     sub atan2($y, $x = 1e0)
     method atan2($x = 1e0)
 
-The sub should usually be written with two arguments for clarity as it is seen in other languages
-and in mathematical texts, but the single-argument form is available.
+The sub should usually be written with two arguments for clarity as it
+is seen in other languages and in mathematical texts, but the
+single-argument form is available; its result will always match that of
+L<atan|/routine/atan>.
 
     say atan2 3, 1              # OUTPUT: «1.2490457723982544␤»
-    say atan2 3                 # OUTPUT: «1.2490457723982544␤»  
+    say atan2 3                 # OUTPUT: «1.2490457723982544␤»
     say atan2 ⅔, ⅓              # OUTPUT: «1.1071487177940904␤»
 
-The method coerces self and its single argument to L<Numeric|/type/Numeric>, using them to compute the
-two-argument L<arc-tangent|https://en.wikipedia.org/wiki/Atan2> in radians.
+The method coerces self and its single argument to
+L<Numeric|/type/Numeric>, using them to compute the two-argument
+L<arc-tangent|https://en.wikipedia.org/wiki/Atan2> in radians.
 
     say 3.atan2;                # OUTPUT: «1.24904577239825␤»
     say ⅔.atan2(⅓);             # OUTPUT: «1.1071487177940904␤»
 
-The $x argument in eiher the method or the sub defaults to 1 so, in both single-argument cases, the function will return
-the angle θ in radians between a vector that goes from the origin to the point (3, 1)
-and the x axis.
+The $x argument in eiher the method or the sub defaults to 1 so, in both
+single-argument cases, the function will return the angle θ in radians
+between the x-axis and a vector that goes from the origin to the point
+(3, 1).
 
 =head2 routine sec