Document MS_WMS_ERROR_STATUS_CODE

[rouault]

added in MapServer/MapServer#7191

[github-actions]

The backport to branch-8-4 failed:

The process '/usr/bin/git' failed with exit code 128

stderr

fatal: invalid reference: branch-8-4

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-branch-8-4 branch-8-4
# Navigate to the new working tree
cd .worktrees/backport-branch-8-4
# Create a new branch
git switch --create backport-985-to-branch-8-4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick c6ea47c0208747926edb8adaf9f5ff30c38fa21b
# Push it to GitHub
git push --set-upstream origin backport-985-to-branch-8-4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-branch-8-4

Then, create a pull request where the base branch is branch-8-4 and the compare/head branch is backport-985-to-branch-8-4.