docs: Update SitemapRequestLoader documentation (#1520)

- based on #1516

Author: janbuchar

docs/guides/architecture_overview.mdx

@@ -291,7 +291,7 @@ Request loaders provide a subset of <ApiLink to="class/RequestQueue">`RequestQue
 
 - <ApiLink to="class/RequestLoader">`RequestLoader`</ApiLink> - Base interface for read-only access to a stream of requests, with capabilities like fetching the next request, marking as handled, and status checking.
 - <ApiLink to="class/RequestList">`RequestList`</ApiLink> - Lightweight in-memory implementation of `RequestLoader` for managing static lists of URLs.
-- <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> - Specialized loader for reading URLs from XML sitemaps with filtering capabilities.
+- <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> - A specialized loader that reads URLs from XML and plain-text sitemaps following the [Sitemaps protocol](https://www.sitemaps.org/protocol.html) with filtering capabilities.
 
 ### Request managers
 

docs/guides/request_loaders.mdx

@@ -31,7 +31,7 @@ The [`request_loaders`](https://github.com/apify/crawlee-python/tree/master/src/
 And specific request loader implementations:
 
 - <ApiLink to="class/RequestList">`RequestList`</ApiLink>: A lightweight implementation for managing a static list of URLs.
-- <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink>: A specialized loader that reads URLs from XML sitemaps with filtering capabilities.
+- <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink>: A specialized loader that reads URLs from XML and plain-text sitemaps following the [Sitemaps protocol](https://www.sitemaps.org/protocol.html) with filtering capabilities.
 
 Below is a class diagram that illustrates the relationships between these components and the <ApiLink to="class/RequestQueue">`RequestQueue`</ApiLink>:
 
@@ -130,7 +130,13 @@ To enable persistence, provide `persist_state_key` and optionally `persist_reque
 
 ### Sitemap request loader
 
-The <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> is a specialized request loader that reads URLs from XML sitemaps. It's particularly useful when you want to crawl a website systematically by following its sitemap structure. The loader supports filtering URLs using glob patterns and regular expressions, allowing you to include or exclude specific types of URLs. The <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> provides streaming processing of sitemaps, ensuring efficient memory usage without loading the entire sitemap into memory.
+The <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> is a specialized request loader that reads URLs from sitemaps following the [Sitemaps protocol](https://www.sitemaps.org/protocol.html). It supports both XML and plain text sitemap formats. It's particularly useful when you want to crawl a website systematically by following its sitemap structure.
+
+:::note
+The `SitemapRequestLoader` is designed specifically for sitemaps that follow the standard Sitemaps protocol. HTML pages containing links are not supported by this loader - those should be handled by regular crawlers using the `enqueue_links` functionality.
+:::
+
+The loader supports filtering URLs using glob patterns and regular expressions, allowing you to include or exclude specific types of URLs. The <ApiLink to="class/SitemapRequestLoader">`SitemapRequestLoader`</ApiLink> provides streaming processing of sitemaps, ensuring efficient memory usage without loading the entire sitemap into memory.
 
 <RunnableCodeBlock className="language-python" language="python">
     {SitemapExample}

src/crawlee/request_loaders/_sitemap_request_loader.py

@@ -90,6 +90,11 @@ class SitemapRequestLoaderState(BaseModel):
 class SitemapRequestLoader(RequestLoader):
     """A request loader that reads URLs from sitemap(s).
 
+    The loader is designed to handle sitemaps that follow the format described in the Sitemaps protocol
+    (https://www.sitemaps.org/protocol.html). It supports both XML and plain text sitemap formats.
+    Note that HTML pages containing links are not supported - those should be handled by regular crawlers
+    and the `enqueue_links` functionality.
+
     The loader fetches and parses sitemaps in the background, allowing crawling to start
     before all URLs are loaded. It supports filtering URLs using glob and regex patterns.