- Included detailed documentation in _sre.c explaining how, when, and why

- Included detailed documentation in _sre.c explaining how, when, and why
  to use LASTMARK_SAVE()/LASTMARK_RESTORE(), based on the discussion
  in patch #712900.

- Cleaned up LASTMARK_SAVE()/LASTMARK_RESTORE() usage, based on the
  established rules.

- Moved the upper part of the just commited patch (relative to bug #725106)
  to outside the for() loop of BRANCH OP. There's no need to mark_save()
  in every loop iteration.