docs(api/filter): before_post_render & after_post_render process all files

[curbengh]

Closes #581

Check List

Please read and check followings before submitting a PR.

• [ ] I want to publish my theme on Hexo official website.
  • [ ] I have read the theme publishing doc.
  • [ ] name is unique.
  • [ ] link URL is correct.
  • [ ] preview URL is correct.
  • [ ] preview URL web site is rendered correctly.
  • [ ] Add a screenshot to source/themes/screenshots.
  • [ ] Screenshot filename is same as value of name.
  • [ ] Screenshot size is 800 * 500.
  • [ ] Screenshot file format is png.
• [ ] I want to publish my plugin on Hexo official website.
  • [ ] I have read the plugin publishing doc.
  • [ ] name is unique.
  • [ ] link URL is correct.
• [x] Others (Update, fix, translation, etc...)
  • Languages:
  • [x] en English
  • [x] ko Korean
  • [x] pt-br Brazilian Portuguese
  • [x] ru Russian
  • [x] th Thai
  • [x] zh-cn simplified Chinese
  • [x] zh-tw traditional Chinese