Modify S6698&S6703&S6697: Add a warning against default values (#3121)

## Review

A dedicated reviewer checked the rule description successfully for:

- [ ] logical errors and incorrect information
- [ ] information gaps and missing content
- [ ] text style and tone
- [ ] PR summary and labels follow [the
guidelines](https://github.com/SonarSource/rspec/#to-modify-an-existing-rule)

---------

Co-authored-by: daniel-teuchert-sonarsource <141642369+daniel-teuchert-sonarsource@users.noreply.github.com>

rules/S6697/secrets/rule.adoc

@@ -38,6 +38,8 @@ They can be used to audit malicious use of password-leak-affected accounts.
 
 include::../../../shared_content/secrets/fix/vault.adoc[]
 
+include::../../../shared_content/secrets/fix/default.adoc[]
+
 === Code examples
 
 ==== Noncompliant code example

rules/S6698/secrets/rule.adoc

@@ -39,6 +39,8 @@ for this to happen.
 
 include::../../../shared_content/secrets/fix/vault.adoc[]
 
+include::../../../shared_content/secrets/fix/default.adoc[]
+
 === Code examples
 
 ==== Noncompliant code example

rules/S6703/secrets/rule.adoc

@@ -33,6 +33,8 @@ include::../../../shared_content/secrets/fix/revoke.adoc[]
 
 include::../../../shared_content/secrets/fix/vault.adoc[]
 
+include::../../../shared_content/secrets/fix/default.adoc[]
+
 === Code examples
 
 ==== Noncompliant code example

shared_content/secrets/fix/default.adoc

@@ -0,0 +1,16 @@
+**Never hard-code secrets, not even the default values**
+
+It is important that you do not hard-code secrets, even default values.
+
+First, hard-coded default secrets are often short and can be easily compromised
+even by attackers who do not have access to the code base.
+
+Second, hard-coded default secrets can cause problems if they need to be
+changed or replaced.
+
+And most importantly, there is always the possibility to accidentally set
+default secrets for production services, which can lead to security
+vulnerabilities and make production insecure by default.
+
+To minimize these risks, it is recommended to apply the above strategies, even
+for the default settings.