Update QUIC_CREDENTIAL_CONFIG.md

[ladnir]

Updated the doc to let the user know QUIC_CREDENTIAL_FLAG_USE_TLS_BUILTIN_CERTIFICATE_VALIDATION must be used if they want to add a custom CA.

Description

This was difficult to find out manually and it does not appear to be documented elsewhere.

Testing

N/A

Documentation

N/A

[qzhuyan]

same issue for me on macOS only.

docs says it should be default for posix QUIC_CREDENTIAL_FLAG_USE_TLS_BUILTIN_CERTIFICATE_VALIDATION

https://github.com/microsoft/msquic/blob/a11b58bc6af845eede2175006df4b0603deb41b2/docs/api/QUIC_CREDENTIAL_CONFIG.md?plain=1#L108