Add missing 'cond-expand' feature identifiers; remove srfi-6 from core list.

Add missing 'cond-expand' feature identifiers; remove srfi-6 from core list.

* module/ice-9/boot-9.scm (%cond-expand-features): Remove redundant list
  of feature identifiers in the comment.  Explain more clearly what
  belongs in this list.  Remove srfi-6.

* module/srfi/srfi-4.scm, module/srfi/srfi-27.scm,
  module/srfi/srfi-31.scm, module/srfi/srfi-38.scm,
  module/srfi/srfi-39.scm, module/srfi/srfi-42.scm,
  module/srfi/srfi-45.scm, module/srfi/srfi-67.scm: Add missing
  'cond-expand-provide'.

* module/srfi/srfi-69.scm: Fix erroneous 'cond-expand-provide'.

* doc/ref/srfi-modules.texi (SRFI-0): Update the list of features in
  Guile core.