Updated MultipartEntityBuilder javadocs clarifying content validation and sanitization as a responsibility of the caller

Author: ok2c

httpclient5/src/main/java/org/apache/hc/client5/http/entity/mime/MultipartEntityBuilder.java

@@ -45,6 +45,11 @@
 
 /**
  * Builder for multipart {@link HttpEntity}s.
+ * <p>
+ * IMPORTANT: it is responsibility of the caller to validate / sanitize content of body
+ * parts, for instance, to ensure they do not contain the boundary value that can prevent
+ * the consumer of the entity from correctly parsing / processing the body parts.
+ * </p>
  *
  * @since 5.0
  */