etag-save.md: mention how using both options is a good idea

Ref: https://curl.se/mail/archive-2025-04/0011.html

Closes #17217

diff --git a/docs/cmdline-opts/etag-save.md b/docs/cmdline-opts/etag-save.md
index 237b5dc54873c04e29d56629561801f2df29d5d9..137a4d5a55f63996038d989d789b180a3c808954 100644 (file)
--- a/docs/cmdline-opts/etag-save.md
+++ b/docs/cmdline-opts/etag-save.md
@@ -21,5 +21,10 @@ usually returned in a response. Use this option with a single URL only.
 
 If no ETag is sent by the server, an empty file is created.
 
+In many situations you want to use an existing etag in the request to avoid
+downloading the same resource again but also save the new etag if it has
+indeed changed, by using both etag options --etag-save and --etag-compare with
+the same filename, in the same command line.
+
 Starting in curl 8.12.0, using the --create-dirs option can also create
 missing directory components for the path provided in --etag-save.