issue #2607, enable '\e' character escape for --standard cc65

doc/cc65.sgml

@@ -1053,6 +1053,16 @@ This cc65 version has some extensions to the ISO C standard.
         unsigned char foo = 0b101; // sets it to 5
         </verb></tscreen>
 
+<item>  The character escape '\e', a GCC C extension, is accepted.
+        In ASCII this is the escape character 0x1B, which may be
+        remapped in other character sets via a #pragma charmap.
+        It can be disabled with the <tt><ref id="option--standard"
+        name="--standard"></tt> option.
+
+        <tscreen><verb>
+        unsigned char foo = '\e'; // sets it to 0x1B or equivalent
+        </verb></tscreen>
+
 </itemize>
 <p>