Tweak fxn behavior description

[srajiang]

Summary

There's a lack of clarity in the behavior described for directMention middleware built-in, as per #984. Updates the description in English. Japanese language change suggestion by @jiangem.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #989 (b305e6c) into main (6acdcfb) will not change coverage.
The diff coverage is n/a.

❗ Current head b305e6c differs from pull request most recent head a34951b. Consider uploading reports for the commit a34951b to get more accurate results

@@           Coverage Diff           @@
##             main     #989   +/-   ##
=======================================
  Coverage   66.25%   66.25%           
=======================================
  Files          13       13           
  Lines        1212     1212           
  Branches      357      357           
=======================================
  Hits          803      803           
  Misses        339      339           
  Partials       70       70           

---

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 6acdcfb...a34951b. Read the comment docs.

[seratch]

Added a minor suggestion on the Japanese language change.