Mention workaround to change margins

[積丹尼 Dan Jacobson]

(info "(texinfo) @pagesizes") says:

      To make more elaborate changes, such as changing any of the page
   margins, you must define a new command in 'texinfo.tex' or
   'texinfo.cnf'.

Mention a workaround:

Before:
$ perl -wle 'for(1..2){print "ABC " x 33, "\n";}'|makeinfo --plaintext
ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC
ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC

   ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC
ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC

After:
$ perl -wle 'for(1..2){print "ABC " x 33, "\n";}'|makeinfo --plaintext 
--fill-column=69|sed 's/^/   /'
   ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC
   ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC

      ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC
   ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC ABC