doc: add rem document

[wg5945]

Please makes sure the items are checked before submitting your PR, thank you!

• Rebase before creating a PR to keep commit history clear.
• Only One commit
• No eslint errors

[codecov-io]

Codecov Report

Merging #3440 into v2 will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##               v2    #3440      +/-   ##
==========================================
- Coverage   19.35%   19.31%   -0.04%     
==========================================
  Files         226      226              
  Lines        5601     5601              
  Branches      711      711              
==========================================
- Hits         1084     1082       -2     
- Misses       4398     4401       +3     
+ Partials      119      118       -1

Impacted Files	Coverage Δ
src/components/qrcode/index.vue	4.54% <0%> (-4.55%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1019808...ee83138. Read the comment docs.