Update API README to be clearer and contain section on rendering docs. (#1091)

Author: AndrewJakubowicz

packages/lit-dev-api/README.md

@@ -1,27 +1,29 @@
 # `lit-dev-api`
 
-This package contains the necessary data for generating API documentation on
-Lit.dev.
+Contains necessary data for generating Lit.dev API docs.
 
-## Updating API Documentation
+## Update Generated API Documentation
 
-1. Locate the configuration file for the relevant package that you want to
-   update (e.g., `packages/lit-dev-tools-cjs/src/api-docs/configs/lit-2.ts`).
-2. Update the `commit` field with the desired commit hash.
-3. Run the following command: `npm run build --workspace lit-dev-api`.
+1. Update the `commit` field with the desired commit hash, within
+   `packages/lit-dev-tools-cjs/src/api-docs/configs/lit-2.ts`.
+1. Execute `npm run build --workspace lit-dev-api`.
 
-Upon completion, the updated data will be reflected in the `pages.json` and
+Updated API documentation will be reflected in the `pages.json` and
 `symbols.json` files.
 
 ## Archived Documentation
 
-The following directories contain archived documentation and are no longer
-automatically updated by the process above:
+`lit-html-1` and `lit-element-2` contain archived documentation and are no
+longer automatically updated. To update the archived documentation locally,
+check out a version of Lit.dev prior to [Pull Request
+#1087](https://github.com/lit/lit.dev/pull/1087), and follow the steps listed
+above.
 
-- `lit-html-1`
-- `lit-element-2`
+## How to update Lit.dev API page templates
 
-Both directories include static `pages.json` and `symbols.json` files. To update
-the archived documentation locally, check out a version of Lit.dev prior to
-[Pull Request #1087](https://github.com/lit/lit.dev/pull/1087), and follow the
-steps listed above.
+For `lit-html-1` and `lit-element-2`, the data from `pages.json` and
+`symbols.json` are used with the `api-v1.html` template to generate the
+resulting HTML. Otherwise modify `api.html` for `lit-2`.
+
+Hint: Use `{{ node | dump }}` to debug the structure of the object in
+the template.