doc: add release note for math/big.Rat.FloatPrec

For #50489.

Change-Id: I4544a24327196eb3ed62af64ae5ddb1f60441d12
Reviewed-on: https://go-review.googlesource.com/c/go/+/546357
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Bypass: Robert Griesemer <[email protected]>

Author: griesemer

doc/go1.22.html

@@ -437,12 +437,10 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 
 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
   <dd>
-    <p><!-- https://go.dev/issue/50489 -->
-      TODO: <a href="https://go.dev/issue/50489">https://go.dev/issue/50489</a>: Rat: add FloatPrec() (int, bool)
-    </p>
-
-    <p><!-- CL 539299 -->
-      TODO: <a href="https://go.dev/cl/539299">https://go.dev/cl/539299</a>: math/big: implement Rat.FloatPrec; modified api/next/50489.txt
+    <p><!-- https://go.dev/issue/50489, CL 539299 -->
+      The new method <a href="/pkg/math/big#Rat.FloatPrec">Rat.FloatPrec</a> computes the number of fractional decimal digits
+      required to represent a rational number accurately as a floating-point number, and whether accurate decimal representation
+      is possible in the first place.
     </p>
   </dd>
 </dl><!-- math/big -->