Remove ambiguity in buffersize description.

Previously it was a bit ambigious to say
"pass this to fully utilize pool's capacity" as the fastest
way would still be not passing `buffersize` at all.
Now this section clearly says "... when using this feature".

Author: obaltian

Doc/library/multiprocessing.rst

@@ -2466,9 +2466,10 @@ with the :class:`Pool` class.
       *buffersize* is specified to limit the number of submitted tasks whose
       results have not yet been yielded. If the buffer is full, iteration over
       the *iterables* pauses until a result is yielded from the buffer.
-      To fully utilize pool's capacity, set *buffersize* to the number of
-      processes in pool (to consume *iterable* as you go) or even higher
-      (to prefetch *buffersize - processes* arguments).
+      To fully utilize pool's capacity when using this feature,
+      set *buffersize* at least to the number of processes in pool
+      (to consume *iterable* as you go), or even higher
+      (to prefetch the next ``N=buffersize-processes`` arguments).
 
       .. versionadded:: 3.15
          Added the *buffersize* parameter.

Misc/NEWS.d/next/Library/2025-07-28-00-37-22.gh-issue-64192.iCGeQ5.rst renamed to Misc/NEWS.d/next/Library/2025-07-28-11-00-49.gh-issue-64192.7htLtg.rst

@@ -3,6 +3,7 @@ Add the optional ``buffersize`` parameter to
 :meth:`multiprocessing.pool.Pool.imap_unordered` to limit the number of
 submitted tasks whose results have not yet been yielded. If the buffer is
 full, iteration over the *iterables* pauses until a result is yielded from
-the buffer. To fully utilize pool's capacity, set *buffersize* to the number
-of processes in pool (to consume *iterable* as you go) or even higher (to
-prefetch *buffersize - processes* arguments).
+the buffer. To fully utilize pool's capacity when using this feature, set
+*buffersize* at least to the number of processes in pool (to consume
+*iterable* as you go), or even higher (to prefetch the next
+``N=buffersize-processes`` arguments).