Update docs for non aws-config config builders (#3960)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
Partially addresses awslabs/aws-sdk-rust#1226,
although we might want to look into going further and actually
deprecating or warning on these builder methods.

## Description
<!--- Describe your changes in detail -->

## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Doc only update

----

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codegen-client/src/main/kotlin/software/amazon/smithy/rust/codegen/client/smithy/generators/config/ServiceConfigGenerator.kt

@@ -399,6 +399,13 @@ class ServiceConfigGenerator(
         }
 
     fun render(writer: RustWriter) {
+        val configDocs = """
+            Constructs a config builder.
+            <div class="warning">
+            Note that a config created from this builder will not have the same safe defaults as one created by
+            the <a href="https://crates.io/crates/aws-config" target="_blank">aws-config</a> crate.
+            </div>
+        """
         writer.docs("Configuration for a $moduleUseName service client.\n")
         customizations.forEach {
             it.section(ServiceConfig.ConfigStructAdditionalDocs)(writer)
@@ -424,9 +431,9 @@ class ServiceConfigGenerator(
         }
 
         writer.rustBlock("impl Config") {
+            writer.docs(configDocs)
             writer.rustTemplate(
                 """
-                /// Constructs a config builder.
                 pub fn builder() -> Builder { Builder::default() }
                 """,
             )
@@ -483,7 +490,7 @@ class ServiceConfigGenerator(
         }
 
         writer.rustBlock("impl Builder") {
-            writer.docs("Constructs a config builder.")
+            writer.docs(configDocs)
             writer.rust("pub fn new() -> Self { Self::default() }")
 
             builderFromConfigBag()(this)