[master] b1d31057d doc: Explain that we do not have partial object caching

[Nils Goroll]

commit b1d31057dd3e00e5ebf1b9d70314d39b52ceb2c8
Author: Nils Goroll <nils.goroll at uplex.de>
Date:   Mon Jan 27 16:15:46 2025 +0100

    doc: Explain that we do not have partial object caching
    
    closes #4256

diff --git a/doc/sphinx/users-guide/storage-backends.rst b/doc/sphinx/users-guide/storage-backends.rst
index 043025c2c..36014ff81 100644
--- a/doc/sphinx/users-guide/storage-backends.rst
+++ b/doc/sphinx/users-guide/storage-backends.rst
@@ -18,6 +18,13 @@ configuration is to use the malloc backend with a limited size. For a
 serious Varnish deployment you probably would want to adjust the storage
 settings.
 
+All built-in storage backends cache full objects only, so, for example, to
+support *n* concurrent cache hits on 1GB sized objects, the storage backend
+should be configured to provide at least *n*\ GB of storage. For uncacheable
+objects, the rule of thumb is *n* x ``transit_buffer``.
+
+Storage backends are also called stevedores.
+
 default
 ~~~~~~~