Add Credential (SecureString) warnings

[Kriegel]

In the SecureString Class documentation, there is a Warning from the platform-compat team, that Securestring shouldn't be used.
Direct Link to the Warning on Github

I am aware of removing SecureString is a breaking change.

So since PowerShell is running on non Windows systems, which perhaps doesn't support SecureString encryption.

I think we have to talk about to implement user warnings into the documentation, where PowerShell Credentials or SecureString is mentioned.

See also #10880