DOC: add few comments to examples.

DOC: add few comments to examples.

- http-check expect rstring is missing html comment closing tag.
- option redis-check: See also "option tcp-check", "tcp-check expect".
- ignore-persist static url example ((from 7.2 Using ACLs to form
  conditions)
- tcp-request content: hdr(x-forwarded-for) example: added
  hdr_ip(x-forwarded-for) version.
- tcp-request content: added comments to sc0_inc_gpc0 stick table example.
- timeout tarpit: mention http-request tarpit.