Clarify case-insensitive nature of HttpHeaders

Closes gh-22723

spring-web/src/main/java/org/springframework/http/HttpHeaders.java

@@ -63,6 +63,8 @@ import org.springframework.util.StringUtils;
  * <li>{@link #set(String, String)} sets the header value to a single string value</li>
  * </ul>
  *
+ * <p>Note that {@code HttpHeaders} generally treats header names in a case-insensitive manner.
+ *
  * @author Arjen Poutsma
  * @author Sebastien Deleuze
  * @author Brian Clozel
@@ -415,6 +417,7 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Construct a new, empty instance of the {@code HttpHeaders} object.
+	 * <p>This is the common constructor, using a case-insensitive map structure.
 	 */
 	public HttpHeaders() {
 		this(CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH)));
@@ -422,6 +425,9 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	/**
 	 * Construct a new {@code HttpHeaders} instance backed by an existing map.
+	 * <p>This constructor is available as an optimization for adapting to existing
+	 * headers map structures, primarily for internal use within the framework.
+	 * @param headers the headers map (expected to operate with case-insensitive keys)
 	 * @since 5.1
 	 */
 	public HttpHeaders(MultiValueMap<String, String> headers) {