Update CodeQL CLI manual (#46039)

Co-authored-by: Sam Browning <106113886+sabrowning1@users.noreply.github.com>

content/code-security/codeql-cli/codeql-cli-manual/database-unbundle.md

@@ -86,3 +86,13 @@ the running subcommand.
 
 (To write a log file with a name you have full control over, instead
 give `--log-to-stderr` and redirect stderr as desired.)
+
+#### `--common-caches=<dir>`
+
+\[Advanced] Controls the location of cached data on disk that will
+persist between several runs of the CLI, such as downloaded QL packs and
+compiled query plans. If not set explicitly, this defaults to a
+directory named `.codeql` in the user's home directory; it will be
+created if it doesn't already exist.
+
+Available since `v2.15.2`.