core/segment.{h,cpp}: gather comments in one place and be more explicit about 'evalue_threshold'

9 jobs for feature-a/3354
Status Job ID Name Coverage
  Test Tools
canceled #134604
test_tools

 
  Test Unit
skipped #134605
test_algo_unit
skipped #134606
test_tools_unit
 
  Test Functional
skipped #134607
test_algo_should
 
  Test Shouldvdj
skipped #134608
test_algo_shouldvdj
 
  Coverage
skipped #134609
algo_coverage
 
  Prepare Release
skipped #134610
allowed to fail manual
prepare_release
 
  Valgrind Unit
skipped #134611
valgrind
algo_valgrind_unit
 
  Valgrind Functional
skipped #134612
valgrind allowed to fail manual
algo_valgrind_functional