CodingGuidelines: reword parameter expansion section

Group entries related to parameter substitutions together and avoid
using the word "regexp" to refer to the ${parameter/pattern/string}
substitution (banned), as the pattern there is a shell glob and not
a regular expression.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/CodingGuidelines

@@ -35,25 +35,28 @@ For shell scripts specifically (not exhaustive):
    properly nests.  It should have been the way Bourne spelled
    it from day one, but unfortunately isn't.
 
+ - We use POSIX compliant parameter substitutions and avoid bashisms;
+   namely:
+
    - We use ${parameter-word} and its [-=?+] siblings, and their
      colon'ed "unset or null" form.
 
    - We use ${parameter#word} and its [#%] siblings, and their
      doubled "longest matching" form.
 
- - We use Arithmetic Expansion $(( ... )).
-
- - Inside Arithmetic Expansion, spell shell variables with $ in front
-   of them, as some shells do not grok $((x)) while accepting $(($x))
-   just fine (e.g. dash older than 0.5.4).
-
    - No "Substring Expansion" ${parameter:offset:length}.
 
    - No shell arrays.
 
    - No strlen ${#parameter}.
 
- - No regexp ${parameter/pattern/string}.
+   - No pattern replacement ${parameter/pattern/string}.
+
+ - We use Arithmetic Expansion $(( ... )).
+
+ - Inside Arithmetic Expansion, spell shell variables with $ in front
+   of them, as some shells do not grok $((x)) while accepting $(($x))
+   just fine (e.g. dash older than 0.5.4).
 
  - We do not use Process Substitution <(list) or >(list).