doc: improve linking to external code docs (#6158)

docs/linking_to_external_code/integrity_checking.md

@@ -1,16 +1,48 @@
 ## Integrity checking & lock files
 
+### Introduction
+
+Let's say your module depends on remote module `https://some.url/a.ts`. When you
+compile your module for the first time `a.ts` is retrieved, compiled and cached.
+It will remain this way until you run your module on a new machine (say in
+production) or reload the cache (through `deno cache --reload` for example). But
+what happens if the content in the remote url `https://some.url/a.ts` is
+changed? This could lead to your production module running with different
+dependency code than your local module. Deno's solution to avoid this is to use
+integrity checking and lock files.
+
+### Caching and lock files
+
 Deno can store and check subresource integrity for modules using a small JSON
 file. Use the `--lock=lock.json` to enable and specify lock file checking. To
-update or create a lock use `--lock=lock.json --lock-write`.
+update or create a lock use `--lock=lock.json --lock-write`. The
+`--lock=lock.json` tells Deno what the lock file to use is, while the
+`--lock-write` is used to output dependency hashes to the lock file
+(`--lock-write` must be used in conjunction with `--lock`).
+
+A `lock.json` might look like this, storing a hash of the file against the
+dependency:
+
+```json
+{
+  "https://deno.land/std@v0.50.0/textproto/mod.ts": "3118d7a42c03c242c5a49c2ad91c8396110e14acca1324e7aaefd31a999b71a4",
+  "https://deno.land/std@v0.50.0/io/util.ts": "ae133d310a0fdcf298cea7bc09a599c49acb616d34e148e263bcb02976f80dee",
+  "https://deno.land/std@v0.50.0/async/delay.ts": "35957d585a6e3dd87706858fb1d6b551cb278271b03f52c5a2cb70e65e00c26a",
+   ...
+}
+```
 
 A typical workflow will look like this:
 
+**src/deps.ts**
+
 ```ts
 // Add a new dependency to "src/deps.ts", used somewhere else.
 export { xyz } from "https://unpkg.com/xyz-lib@v0.9.0/lib.ts";
 ```
 
+Then:
+
 ```shell
 # Create/update the lock file "lock.json".
 deno cache --lock=lock.json --lock-write src/deps.ts
@@ -26,8 +58,28 @@ Collaborator on another machine -- in a freshly cloned project tree:
 ```shell
 # Download the project's dependencies into the machine's cache, integrity
 # checking each resource.
-deno cache -r --lock=lock.json src/deps.ts
+deno cache --reload --lock=lock.json src/deps.ts
 
 # Done! You can proceed safely.
 deno test --allow-read src
 ```
+
+### Runtime verification
+
+Like caching above, you can also use the `--lock=lock.json` option during use of
+the `deno run` sub command, validating the integrity of any locked modules
+during the run. Remember that this only validates against dependencies
+previously added to the `lock.json` file. New dependencies will be cached but
+not validated.
+
+You can take this a step further as well by using the `--cached-only` flag to
+require that remote dependencies are already cached.
+
+```shell
+deno run --lock=lock.json --cached-only mod.ts
+```
+
+This will fail if there are any dependencies in the dependency tree for mod.ts
+which are not yet cached.
+
+<!-- TODO - Add detail on dynamic imports -->