hsts.md: mention multiple curl invokes effect

Reported-by: zhanhb on github
Ref: #21847
Closes #21851

diff --git a/docs/cmdline-opts/hsts.md b/docs/cmdline-opts/hsts.md
index f99b91c281443e2a550edd1401677fcc8aab657f..7653c65cd5d94b71c6a4422910910a36e02be0c4 100644 (file)
--- a/docs/cmdline-opts/hsts.md
+++ b/docs/cmdline-opts/hsts.md
@@ -18,7 +18,9 @@ Example:
 
 Enable HSTS for the transfer. If the filename points to an existing HSTS cache
 file, that is used. After a completed transfer, the cache is saved to the
-filename again if it has been modified.
+filename again if it has been modified. If you run multiple curl invokes at
+the same time using the same HSTS cache file, they might interfere with each
+other in possibly undesired ways.
 
 If curl is told to use `http://` for a transfer involving a hostname that
 exists in the HSTS cache, it upgrades the transfer to use HTTPS. Each HSTS