docs: mention umask need when curl creates files

for cookies, alt-svc and HSTS, command line and library

Closes #19396

docs/libcurl/opts/CURLOPT_ALTSVC.md

@@ -47,6 +47,10 @@ libcurl cannot fully protect against attacks where an attacker has write
 access to the same directory where it is directed to save files. This is
 particularly sensitive if you save files using elevated privileges.
 
+libcurl creates the file to store the alt-svc cache in using default file
+permissions, meaning that on *nix systems you may need to restrict your umask
+to prevent other users on the same system to access the file.
+
 # DEFAULT
 
 NULL. The alt-svc cache is not read nor written to file.

docs/libcurl/opts/CURLOPT_COOKIEJAR.md

@@ -58,6 +58,10 @@ libcurl cannot fully protect against attacks where an attacker has write
 access to the same directory where it is directed to save files. This is
 particularly sensitive if you save files using elevated privileges.
 
+libcurl creates the file to store cookies using default file permissions,
+meaning that on *nix systems you may need to restrict your umask to prevent
+other users on the same system to access the file.
+
 # DEFAULT
 
 NULL

docs/libcurl/opts/CURLOPT_HSTS.md

@@ -67,6 +67,10 @@ libcurl cannot fully protect against attacks where an attacker has write
 access to the same directory where it is directed to save files. This is
 particularly sensitive if you save files using elevated privileges.
 
+libcurl creates the file to store HSTS data in using default file permissions,
+meaning that on *nix systems you may need to restrict your umask to prevent
+other users on the same system to access the file.
+
 # %PROTOCOLS%
 
 # EXAMPLE