doc: fix two id/xreflabel inconsistencies in config.sgml

 Hi hackers,

  Attached is a tiny docs patch that fixes two SGML attribute
  inconsistencies in config.sgml.

  1. The varlistentry for enable_group_by_reordering has
     id="guc-enable-groupby-reordering" — every other enable_*
     parameter in the file uses the guc-enable-X-y-z form that
     mirrors the underscores in the GUC name. Renaming to
     guc-enable-group-by-reordering matches that convention.

  2. The varlistentry for quote_all_identifiers has
     xreflabel="quote-all-identifiers" with hyphens — every other
     xreflabel in the file matches the GUC name verbatim with
     underscores. Fixing to xreflabel="quote_all_identifiers".

  Neither id has any incoming linkend reference, so this is purely a
  consistency change with no callsite churn.

  I used an LLM (Claude Code) as part of preparing this — it spotted
  the inconsistencies during a sweep of config.sgml. I reviewed the
  diff myself before sending.

  Regards,
  Bill Kim