Add note about docs/ja

[tars0x9752]

The added note means as follows:

NOTE: This documentation is a partial translation of the original into Japanese. If you'd like to read the complete documentation and the most up-to-date information, please refer to the original.

[codecov]

Codecov Report

Merging #2353 (de7dab5) into main (d74e70e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2353   +/-   ##
=======================================
  Coverage   91.09%   91.09%           
=======================================
  Files          19       19           
  Lines       10573    10573           
  Branches     1697     1697           
=======================================
  Hits         9631     9631           
  Misses        932      932           
  Partials       10       10           

Flag	Coverage Δ
test262	84.01% <ø> (ø)
tests	81.76% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

---

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 d74e70e...de7dab5. Read the comment docs.

[ptomato]

Thanks, makes sense.