Schema Inaccuracy

c.f. https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#merge-a-pull-request

https://github.com/github/rest-api-description/blob/50bf833eb2c1288fb78419d9e4d359fda3c3ccbe/descriptions/api.github.com/api.github.com.yaml#L36643-L36645

This documents:

commit_message string
Extra detail to append to automatic commit message.

However, it seems like, at least for the "squash" merge method, it replaces the commit_message, rather than appending it.

Expected

I hoped it would, like the behaviour of "Squash & Merge" in the Web UI, allow me to specify the entire commit message.

I fear it would, as documented, keep the auto-generated commit message and append a string there.

Reproduction Steps

gsnedders@gsnedders-lonnie tmp-symmetrical-meme % gh api \             
  --method PUT \
  -H "Accept: application/vnd.github+json" \
  -H "X-GitHub-Api-Version: 2022-11-28" \
  /repos/gsnedders/tmp-symmetrical-meme/pulls/2/merge \
   -f "commit_title=Fix README" -f "commit_message=Will this be appended or replace the old body?" -f "merge_method=squash"

This merge entirely replaced the old body, rather than appended it as documented.