Fixes in sorting descriptions (GH-18317)

Improvements in listsort.txt and a comment in sortperf.py.

Automerge-Triggered-By: @csabella

Lib/test/sortperf.py

@@ -134,7 +134,7 @@ def tabulate(r):
         L = list(range(half - 1, -1, -1))
         L.extend(range(half))
         # Force to float, so that the timings are comparable.  This is
-        # significantly faster if we leave tham as ints.
+        # significantly faster if we leave them as ints.
         L = list(map(float, L))
         doit(L) # !sort
         print()

Objects/listsort.txt

@@ -319,13 +319,13 @@ So merging is always done on two consecutive runs at a time, and in-place,
 although this may require some temp memory (more on that later).
 
 When a run is identified, its base address and length are pushed on a stack
-in the MergeState struct.  merge_collapse() is then called to see whether it
+in the MergeState struct.  merge_collapse() is then called to potentially
-should merge it with preceding run(s).  We would like to delay merging as
+merge runs on that stack.  We would like to delay merging as long as possible
-long as possible in order to exploit patterns that may come up later, but we
+in order to exploit patterns that may come up later, but we like even more to
-like even more to do merging as soon as possible to exploit that the run just
+do merging as soon as possible to exploit that the run just found is still
-found is still high in the memory hierarchy.  We also can't delay merging
+high in the memory hierarchy.  We also can't delay merging "too long" because
-"too long" because it consumes memory to remember the runs that are still
+it consumes memory to remember the runs that are still unmerged, and the
-unmerged, and the stack has a fixed size.
+stack has a fixed size.
 
 What turned out to be a good compromise maintains two invariants on the
 stack entries, where A, B and C are the lengths of the three rightmost not-yet
@@ -739,7 +739,7 @@ slice (leaving off both endpoints) (2**(k-1)-1)+1 through (2**k-1)-1
 inclusive = 2**(k-1) through (2**k-1)-1 inclusive, which has
     (2**k-1)-1 - 2**(k-1) + 1 =
     2**k-1 - 2**(k-1) =
-    2*2**k-1 - 2**(k-1) =
+    2*2**(k-1)-1 - 2**(k-1) =
     (2-1)*2**(k-1) - 1 =
     2**(k-1) - 1
 elements.