Update advopt.txt to include link to nim cache docs (#13464)

doc/advopt.txt

@@ -49,6 +49,7 @@ Advanced options:
   --import:PATH             add an automatically imported module
   --include:PATH            add an automatically included module
   --nimcache:PATH           set the path used for generated files
+                            see also https://nim-lang.org/docs/nimc.html#compiler-usage-generated-c-code-directory 
   -c, --compileOnly:on|off  compile Nim files only; do not assemble or link
   --noLinking:on|off        compile Nim and generated files but do not link
   --noMain:on|off           do not generate a main procedure