Rename workflow to reflect its generic nature

This workflow can (pretty much) be used by any other workflow that wants
to post a PR comment.

Author: hmac

.github/workflows/post-pr-comment.yml

@@ -2,7 +2,7 @@
 # .github/workflows/qhelp-pr-preview.yml
 # See that file for more info.
 
-name: Add PR comment with QHelp preview
+name: Post PR comment
 on:
   workflow_run:
     workflows: [Render QHelp changes]

.github/workflows/qhelp-pr-preview.yml

@@ -5,7 +5,7 @@
 # It also writes the PR number to `pr_number.txt`.
 # These three files are uploaded as an artifact.
 
-# When this workflow completes, the workflow "Add PR comment with QHelp preview" runs.
+# When this workflow completes, the workflow "Post PR comment" runs.
 # It downloads the artifact and adds a comment to the PR with the rendered
 # QHelp.