Mention the non-conformance of the core SRFI-6 procedures in the manual.

Mention the non-conformance of the core SRFI-6 procedures in the manual.

* doc/ref/srfi-modules.texi (About SRFI Usage): Use SRFI-13 as an
  example of a module which is included in core Guile, instead of
  SRFI-6.

  (SRFI-6): Mention the non-conformance of the core 'open-input-string'
  and 'open-output-string' procedures.  Remove the claim that importing
  this module does nothing.  Recommend that users import the module.