Update docs

Author: camdecoster

CONTRIBUTING.md

@@ -171,23 +171,30 @@ compliance, and so for strict builds we pre-generate regl shader code here.
 The CI pipeline will automatically detect when regl-related files have changed and
 run the codegen process. If the precompiled shaders are out of date, the
 `check-regl-codegen` job will fail and upload a `regl-codegen` artifact containing
-the updated files. To fix this:
+the updated files. The artifact represents the full desired state of the codegen
+output (CI wipes the output directory before regenerating, so any orphaned shader
+files are pruned). To fix this:
 
 1. Download the `regl-codegen` artifact from the failed workflow run
-2. Copy the updated files into your working tree
+2. Delete `src/generated/regl-codegen/` in your working tree, then unzip the
+   artifact at the repo root so it replaces (rather than merges into) the
+   existing directory. Also overwrite the four `regl_precompiled.js` files
+   under `src/traces/{scattergl,scatterpolargl,splom,parcoords}/`.
 3. Commit and push the changes to your pull request
 
-Alternatively, you can regenerate the code locally by running:
+Alternatively, you can regenerate the code locally:
 
 ```bash
+rm -rf src/generated/regl-codegen/*
 npm run regl-codegen
 ```
 
-This will prompt you to open a browser window, run through all traces with 'regl'
-in the tags, and store the captured code into
+The `rm -rf` mirrors what CI does, so any orphaned shader files left over from
+previous changes get cleaned up. `npm run regl-codegen` will prompt you to open a
+browser window, run through all traces with 'regl' in the tags, and store the
+captured code into
 [src/generated/regl-codegen](https://github.com/plotly/plotly.js/blob/master/src/generated/regl-codegen).
-If no updates are necessary, it will be a no-op, but if there are changes, you
-will need to commit them.
+Commit any changes that result.
 
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