describe-external-formats in the documentation.

o Document LIST-ALL-EXTERNAL-FORMATS and DESCRIBE-EXTERNAL-FORMAT.

little information about the format.  Provide a means to get a list of
external formats and to display the documentation.

bootfiles/20a/boot-2010-07-1.lisp:
o Use this bootstrap file when doing a normal build.

code/exports.lisp:
o New functions:
  - Add LIST-ALL-EXTERNAL-FORMATS to list all available external formats
    and their corresponding aliases.
  - Add DESCRIBE-EXTERNAL-FORMAT to print some information about the
    given format.
o Add docuemntation slot to defstruct EXTERNAL-FORMAT.
o Change DEFINE-EXTERNAL-FORMAT macro.  Adds :DOCUMENTATION keyword to
  specify the documentation.  Add :BASE keyword indicate that the
  external format is based on another format.  (Previously, this
  wasn't needed, but is somewhat incompatible with adding a
  documentation string.)
o Change DEFINE-COMPOSING-EXTERNAL-FORMAT to include :documentation
  keyword to specify the documentation for the format.
o Minor reindentation of some docstrings.
o Make sure documentation strings for external format are marked for
  translation; wrap other strings with intl:gettext to explicitly mark
  them for translations.
o Add docstring for VOID and ISO8859-1 external formats.

code/exports.lisp:
o Export the new symbols LIST-ALL-EXTERNAL-FORMATS and
  DESCRIBE-EXTERNAL-FORMAT.  Import into EXTENSIONS package.

docs/cmu-user/unicode.tex:
o Update docs to include LIST-ALL-EXTERNAL-FORMATS and
  DESCRIBE-EXTERNAL-FORMAT.
o Update docs for DEFINE-EXTERNAL-FORMAT and
  DEFINE-COMPOSING-EXTERNAL-FORMAT to match implementation.

general-info/release-20b.txt:
o Update

external-formats/*.lisp:
o Update with docstrings.
o Add :BASE keyword where needed.

o Fix some typos in keyword values for the :CASING argument to
  STRING-UPCASE and friends.
o Make the documentation for OCTETS-TO-STRING match the
  implementation.

unicode-utf16-extfmt-2009-06-11.