docs(Modal): update props' description

[bonellia]

I suppose Modal is just a specialized Portal (or vice-versa?) but I suppose props docs for modal could be less ambiguous with these small changes.

[welcome]

💖 Thanks for opening this pull request! 💖

Here is a list of things that will help get it across the finish line:

• Run yarn lint locally to catch formatting errors. This will fix some errors automatically, commit and push any changes.
• Run yarn test locally to catch errors. This ensures all components still behave as they should.
• Run yarn start to run the doc site locally and try a few pages, ensuring everything is in good working order.
• Include tests when adding/changing behavior.

We get a lot of pull requests on this repo, so please be patient and we will get back to you as soon as we can.

[codecov-commenter]

Codecov Report

Merging #4291 (e09c9e7) into master (848f497) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4291   +/-   ##
=======================================
  Coverage   99.75%   99.75%           
=======================================
  Files         180      180           
  Lines        3241     3241           
=======================================
  Hits         3233     3233           
  Misses          8        8           

Impacted Files	Coverage Δ
src/modules/Modal/Modal.js	100.00% <ø> (ø)

---

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 848f497...e09c9e7. Read the comment docs.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉