Re-word the intro slightly to avoid reader misunderstanding: strings are not

Re-word the intro slightly to avoid reader misunderstanding: strings are not
mutable!  We do not want to shock anyone.
This closes SF bug #483805.

Re-factor so that the description of the "access" keyword parameter is not
repeated in both the descriptions of mmap().  Also, only make sure the first
description of mmap() appears in the index.  The the index link is followed,
the first is now used to locate the page on the screen; chances are really good
both will be visible.  This avoids the problem that the index entry for the
second is selected and the first version is not visible, making the reader
consider that mmap() is not available on Windows.