MANUAL: Under security, added note about +RTS option to limit heap size.

See #5047.

MANUAL.txt

@@ -4885,6 +4885,9 @@ application, here are some things to keep in mind:
 3. Pandoc's parsers can exhibit pathological performance on some
    corner cases.  It is wise to put any pandoc operations under
    a timeout, to avoid DOS attacks that exploit these issues.
+   If you are using the pandoc executable, you can add the
+   command line options `+RTS -M512M -RTS` (for example) to limit
+   the heap size to 512MB.
 
 4. The HTML generated by pandoc is not guaranteed to be safe.
    If `raw_html` is enabled for the Markdown input, users can