docs(readme): update description of PAGO_SOCK

Author: dbohdan

README.md

@@ -297,9 +297,10 @@ cap_mkdb /etc/login.conf
   The default character pattern (regular expression) for random passwords
 - `PAGO_SOCK`:
   The agent socket path.
-  Defaults to:
-    - Linux and BSD: `~/.cache/pago/socket`
-    - macOS: `~/Library/Caches/pago/socket`
+  Defaults to a path inside the user runtime directory.
+  This is `${XDG_RUNTIME_DIR}/pago/socket` or `/run/user/<UID>/pago/socket` on Linux.
+  If the primary runtime directory is not available, pago tries `/var/run/user/<UID>/` as a fallback.
+  If that also fails, it uses a user-specific subdirectory in the temporary directory, for example, `/tmp/pago-${USER}/socket`.
 - `PAGO_TIMEOUT`:
   The default timeout to clear the clipboard