bpo-32798: Add restriction on the offset parameter for mmap.flush in the docs (GH-5621)

Add restriction on the offset parameter for mmap.flush.

Explain that ALLOCATIONGRANULARITY is the same as PAGESIZE in Unix.
(cherry picked from commit 027664a3)
Co-authored-by: Pablo Galindo <Pablogsal@gmail.com>

Doc/library/mmap.rst

@@ -68,7 +68,7 @@ memory but does not update the underlying file.
 
    *offset* may be specified as a non-negative integer offset. mmap references
    will be relative to the offset from the beginning of the file. *offset*
-   defaults to 0.  *offset* must be a multiple of the ALLOCATIONGRANULARITY.
+   defaults to 0.  *offset* must be a multiple of the :const:`ALLOCATIONGRANULARITY`.
 
 
 .. class:: mmap(fileno, length[, flags[, prot[, access[, offset]]]])
@@ -97,8 +97,8 @@ memory but does not update the underlying file.
 
    *offset* may be specified as a non-negative integer offset. mmap references
    will be relative to the offset from the beginning of the file. *offset*
-   defaults to 0.  *offset* must be a multiple of the PAGESIZE or
-   ALLOCATIONGRANULARITY.
+   defaults to 0. *offset* must be a multiple of :const:`ALLOCATIONGRANULARITY`
+   which is equal to :const:`PAGESIZE` on Unix systems.
 
    To ensure validity of the created memory mapping the file specified
    by the descriptor *fileno* is internally automatically synchronized
@@ -171,7 +171,8 @@ memory but does not update the underlying file.
       use of this call there is no guarantee that changes are written back before
       the object is destroyed.  If *offset* and *size* are specified, only
       changes to the given range of bytes will be flushed to disk; otherwise, the
-      whole extent of the mapping is flushed.
+      whole extent of the mapping is flushed.  *offset* must be a multiple of the
+      :const:`PAGESIZE` or :const:`ALLOCATIONGRANULARITY`.
 
       **(Windows version)** A nonzero value returned indicates success; zero
       indicates failure.