Comments reformatting in mark.c to properly delimit sentences

Comments reformatting in mark.c to properly delimit sentences
(code refactoring)

* mark.c (GC_clear_hdr_marks, clear_marks_for_block, GC_set_mark_bit,
GC_clear_marks, GC_initiate_gc, GC_mark_from, alloc_mark_stack,
GC_push_all, GC_push_all_eager, GC_push_marked1, GC_push_marked2,
GC_push_marked4): Add training dot to comment sentences (where missing);
capitalize the first letter of comment sentences (where needed);
reformat some comments.
* mark.c [WRAP_MARK_SOME] (GC_mark_some): Likewise.
* mark.c [PARALLEL_MARK] (GC_do_parallel_mark, GC_help_wanted,
GC_helper_count, GC_active_count, GC_steal_mark_stack, GC_mark_local):
Likewise.
* mark.c [!GC_DISABLE_INCREMENTAL] (GC_push_next_marked_dirty):
Likewise.