doc/eval: fix/review the section about SI prefixes and usage

In particular, prefer "prefix" to "postfix" as in the tool manuals, and
specify powers of 2 and 10 explicitly.

This is based on the commit:
commit 2bf794b6
Author: Marcus Stollsteimer <sto.mar@web.de>
Date:   Mon Nov 19 21:39:20 2012 +0100

doc/eval.texi

@@ -209,55 +209,56 @@ In your C code, you can extend the list of unary and binary functions,
 and define recognized constants, so that they are available for your
 expressions.
 
-The evaluator also recognizes the International System number
-postfixes. If 'i' is appended after the postfix, powers of 2 are used
-instead of powers of 10. The 'B' postfix multiplies the value for 8,
-and can be appended after another postfix or used alone. This allows
-using for example 'KB', 'MiB', 'G' and 'B' as postfix.
-
-Follows the list of available International System postfixes, with
+The evaluator also recognizes the International System unit prefixes.
+If 'i' is appended after the prefix, binary prefixes are used, which
+are based on powers of 1024 instead of powers of 1000.
+The 'B' postfix multiplies the value by 8, and can be appended after a
+unit prefix or used alone. This allows using for example 'KB', 'MiB',
+'G' and 'B' as number postfix.
+
+The list of available International System prefixes follows, with
 indication of the corresponding powers of 10 and of 2.
 @table @option
 @item y
--24 / -80
+10^-24 / 2^-80
 @item z
--21 / -70
+10^-21 / 2^-70
 @item a
--18 / -60
+10^-18 / 2^-60
 @item f
--15 / -50
+10^-15 / 2^-50
 @item p
--12 / -40
+10^-12 / 2^-40
 @item n
--9 / -30
+10^-9 / 2^-30
 @item u
--6 / -20
+10^-6 / 2^-20
 @item m
--3 / -10
+10^-3 / 2^-10
 @item c
--2
+10^-2
 @item d
--1
+10^-1
 @item h
-2
+10^2
 @item k
-3 / 10
+10^3 / 2^10
 @item K
-3 / 10
+10^3 / 2^10
 @item M
-6 / 20
+10^6 / 2^20
 @item G
-9 / 30
+10^9 / 2^30
 @item T
-12 / 40
+10^12 / 2^40
 @item P
-15 / 40
+10^15 / 2^40
 @item E
-18 / 50
+10^18 / 2^50
 @item Z
-21 / 60
+10^21 / 2^60
 @item Y
-24 / 70
+10^24 / 2^70
 @end table
 
 @c man end