docs: Add `$` to the escape sequences section

feat: Add the `$` character to the escape sequences table.

doc/17-language-reference.md

@@ -97,6 +97,7 @@ Character                 | Escape sequence
 --------------------------|------------------------------------
 "                         | \\"
 \\                        | \\\\
+$                         | $$
 <TAB>               | \\t
 <CARRIAGE-RETURN>   | \\r
 <LINE-FEED>         | \\n
@@ -107,6 +108,10 @@ In addition to these pre-defined escape sequences you can specify
 arbitrary ASCII characters using the backslash character (\\) followed
 by an ASCII character in octal encoding.
 
+In Icinga 2, the `$` character is reserved for resolving [runtime macros](03-monitoring-basics.md#runtime-macros).
+However, in situations where a string that isn't intended to be used as a runtime macro contains the `$` character,
+it is necessary to escape it with another `$` character.
+
 ### Multi-line String Literals <a id="multiline-string-literals"></a>
 
 Strings spanning multiple lines can be specified by enclosing them in