Document the (hard-coded) maximum HTTP/2 frame size accepted from servers

docs/topics/settings.rst

@@ -689,10 +689,15 @@ The default HTTPS handler uses HTTP/1.1. To use HTTP/2 update
     Scrapy currently does not support HTTP/2 Cleartext (h2c) since none
     of the major browsers support HTTP/2 unencrypted (refer `http2 faq`_).
 
+    Also, Scrapy does not currently support specifying a maximum `frame size`_
+    larger than the default value, 16384. Connections to servers that send a
+    larger frame will fail.
+
 .. warning:: HTTP/2 support in Scrapy is experimental, and not yet recommended
              for production environments. Future Scrapy versions may introduce
              related changes without a deprecation period or warning.
 
+.. _frame size: https://tools.ietf.org/html/rfc7540#section-4.2
 .. _http2 faq: https://http2.github.io/faq/#does-http2-require-encryption
 
 .. setting:: DOWNLOAD_TIMEOUT