man: clarify quoting of `$` in command lines (#39494)

When the special executable prefix `:` is used, `$$` yield the literal `$$`.

diff --git a/man/systemd.service.xml b/man/systemd.service.xml
index 125acae4d37a44baca24ad24c0753a8a7a887bc1..84dd17920a120cf4b5049d94223886e2bf1e01bb 100644 (file)
--- a/man/systemd.service.xml
+++ b/man/systemd.service.xml
@@ -1485,7 +1485,8 @@ ExecStart=/bin/echo $ONE $TWO $THREE</programlisting>
     <literal>too</literal>.
     </para>
 
-    <para>To pass a literal dollar sign, use <literal>$$</literal>.
+    <para>Unless for commands with the special executable prefix <literal>:</literal>,
+    to pass a literal dollar sign, use <literal>$$</literal>.
     Variables whose value is not known at expansion time are treated
     as empty strings. Note that the first argument (i.e. the program
     to execute) may not be a variable.</para>