Mention capture group for regex pii scrubbing (#15660)

Author: jjbayer

docs/security-legal-pii/scrubbing/advanced-datascrubbing.mdx

@@ -58,6 +58,7 @@ Rules generally consist of three parts:
   - For case-insensitivity, prefix your regex with `(?i)`.
   - If you're trying to use one of the popular regex "IDEs" like [regex101.com](https://regex101.com/) select the Rust dialect.
   - Escape using `\`, e.g. `\*` is a literal `*`. This works for any of the following characters: `\.+*?()|[]{}^$`.
+  - If you want to replace a substring of a specific search string, wrap it in `()` and check "Only replace first capture match".
 
 - _Credit Card Numbers_: Any substrings that look like credit card numbers.
 - _Password Fields_: Any substrings that look like they may contain passwords. Any string that mentions passwords, auth tokens or credentials, any variable that is called `password` or `auth`.