docs: Document command to generate local html coverage report

Author: tpoliaw

docs/how-to/coverage.md

@@ -1,12 +1,30 @@
-
 # How to check code coverage
 
-Code coverage is reported to the command line and to a `cov.xml` file by the command `uv run tox -e tests`. The file is uploaded to the Codecov service in CI.
+## From the command line
+Code coverage is reported to the command line and to a `cov.xml` file by the
+command `uv run tox -e tests`. The file is uploaded to the Codecov service in
+CI.
+
+To view the coverage locally in a view similar to codecov, pytest can generate
+html coverage info.
+
+    ```sh
+    $ uv run tox -e tests -- --cov-report html
+    ```
+
+This will generate html in the `htmlcov` directory and opening
+`htmlcov/index.html` in a browser will show browsable coverage information.
 
-If you are using VSCode IDE, you can also check code coverage by right-clicking on the test icon and selecting `Run with coverage`. This will re-run the test and highlight line numbers in your file:
+## From VSCode
+If you are using VSCode IDE, you can also check code coverage by right-clicking
+on the test icon and selecting `Run with coverage`. This will re-run the test
+and highlight line numbers in your file:
 - Green = covered by tests
 - Red = not covered (and ideally should be tested)
 
-## Adding a Codecov Token
+# Adding a Codecov Token
 
-If the repo is not hosted in DiamondLightSource, then you need to visit `https://app.codecov.io/account/gh/<org_name>/org-upload-token` to generate a token for your org, and store it as a secret named `CODECOV_TOKEN` in `https://github.com/organizations/<org_name>/settings/secrets/actions`
+If the repo is not hosted in DiamondLightSource, then you need to visit
+`https://app.codecov.io/account/gh/<org_name>/org-upload-token` to generate a
+token for your org, and store it as a secret named `CODECOV_TOKEN` in
+`https://github.com/organizations/<org_name>/settings/secrets/actions`