[core] add comment for FastCGI mem use in hctx->rb (#3033)
[core] add comment for FastCGI mem use in hctx->rb (#3033)

x-ref:
  "Memory Growth with PUT and full buffered streams"
  https://redmine.lighttpd.net/issues/3033

lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4