docs(cli): name the update cache path for relocated config dirs

Review round 3. The cache is derived from `configDir()`, so it moves with
`SIM_CONFIG_DIR` like the config and credentials files do - but the docs named
only the `~/.sim` default, sending anyone with a relocated config dir to a file
that is not there.

Author: mzxchandra

apps/docs/content/docs/cli/configuration.mdx

@@ -155,11 +155,13 @@ The notice is skipped entirely when:
   deliberately trails the published one
 - the installed version is a prerelease from the `staging` or `dev` channel
 
-The once-a-day pace comes from a timestamp in `~/.sim/update-check.json`. If
-that file cannot be written — a read-only home in a container, or a `~/.sim`
-left root-owned by an earlier `sudo` install — the pace cannot be remembered,
-so the check runs once per command instead of once per day. It stays bounded by
-the same one-second timeout, and `SIM_NO_UPDATE_CHECK=1` still turns it off.
+The once-a-day pace comes from a timestamp in `update-check.json`, kept beside
+the config and credentials files: `~/.sim/update-check.json` by default, and
+under `SIM_CONFIG_DIR` when that is set. If it cannot be written — a read-only
+home in a container, or a `~/.sim` left root-owned by an earlier `sudo` install
+— the pace cannot be remembered, so the check runs once per command instead of
+once per day. It stays bounded by the same one-second timeout, and
+`SIM_NO_UPDATE_CHECK=1` still turns it off.
 
 Set `npm_config_registry` to ask a mirror instead; its path and query are
 preserved, so a token-authenticated Artifactory or Nexus base works.