[doc] setTrustedCertificates doesn't replace certs

With the previous wording, "Sets the set of ... certificates", I've assumed it always replaces everything in the store (at least on non-iOS).
This change should make it clear that the certificates are added.

(See also https://github.com/dart-lang/sdk/blob/bc2cf88bac44d02a44b9c315a8fd05eca2ecbaa4/runtime/bin/security_context.cc#L181 for the actual implementation).

Closes https://github.com/dart-lang/sdk/pull/46372
https://github.com/dart-lang/sdk/pull/46372

GitOrigin-RevId: 04d6cdece5f707af3e4d74aca983e4025fcc6cde
Change-Id: Ib3d3dcf41b78cb3925947d58e5c980b7bb82f331
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203720
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>
diff --git a/sdk/lib/io/security_context.dart b/sdk/lib/io/security_context.dart
index 953d1e6..d041975 100644
--- a/sdk/lib/io/security_context.dart
+++ b/sdk/lib/io/security_context.dart
@@ -69,8 +69,8 @@
   /// of bytes.
   void usePrivateKeyBytes(List<int> keyBytes, {String? password});
 
-  /// Sets the set of trusted X509 certificates used by [SecureSocket]
-  /// client connections, when connecting to a secure server.
+  /// Add a certificate to the set of trusted X509 certificates
+  /// used by [SecureSocket] client connections.
   ///
   /// [file] is the path to a PEM or PKCS12 file containing X509 certificates,
   /// usually root certificates from certificate authorities. For PKCS12 files,
@@ -90,8 +90,8 @@
   /// ```
   void setTrustedCertificates(String file, {String? password});
 
-  /// Sets the set of trusted X509 certificates used by [SecureSocket]
-  /// client connections, when connecting to a secure server.
+  /// Add a certificate to the set of trusted X509 certificates
+  /// used by [SecureSocket] client connections.
   ///
   /// Like [setTrustedCertificates] but takes the contents of the file.
   void setTrustedCertificatesBytes(List<int> certBytes, {String? password});