simpleperf: update doc on broken DWARF call graph issue

1. Explain reasons causing broken call stacks.
2. Explain methods simpleperf provided to alleviate problems
   caused by different reasons. `--user-buffer-size` and `--remove-gaps`
   are the two new options we want to mention.
3. Also modernize some near topics.

Bug: 270608483
Test: none
Change-Id: I0d9baf8ae8602a6985aff5c1ab38e84da053569c
1 file changed