code: func (f *freelist) release(txid txid) {
  	m := make(pgids, 0)
  	for tid, ids := range f.pending {
  		if tid <= txid {
  			// Move transaction's pending pages to the available freelist.
  			// Don't remove from the cache since the page is still free.
  			m = append(m, ids...)
  			delete(f.pending, tid)
  		}
  	}
  	sort.Sort(m)
  	f.ids = pgids(f.ids).merge(m)
  }
docs: // release moves all page ids for a transaction id (or older) to the freelist.