Fix some errors in the documentation and other minor issues (#42)

Somewhat notable changes in this commit:
- The 'set +r' bugfix (re: 74b41621) is now documented in the
  changelog.
- Missing options have been added to the synopsis section of the
  ksh man page.
- The minor formatting fix from https://github.com/ksh-community/ksh/pull/5
  has been applied to the ksh man page.
- A few fixes from 5e747cfb
  have been applied to the ksh man page.
- The man page fixes from https://github.com/att/ast/pull/353
  have been applied, being:
  - An addition to document the behavior of 'set -H'.
  - A fix for the cd section appending rksh93.
  - A fix for some options being indented too far.
  - Removal of a duplicate section documenting '-D'.
  - Reordering the options for 'set' in alphabetical order.
  - A minor fix for the documentation of 'ksh -i'.

src/cmd/ksh93/data/builtins.c

@@ -940,7 +940,7 @@ const char sh_opthash[] =
 "\n"
 "[+EXIT STATUS?]{"
 	"[+0?Successful completion.]"
-	"[+>0?An error occured.]"
+	"[+>0?An error occurred.]"
 "}"
 "[+SEE ALSO?\bsh\b(1), \balias\b(1)]"
 ;