[CLEANUP] Keep in sync "defaults" support between documentation and code

[CLEANUP] Keep in sync "defaults" support between documentation and code

Hi Willy,

I've made a quick pass on the "defaults" column in the Proxy keywords matrix (chapter 4.1. in the documentation).
This patch resyncs the code and the documentation. I let you decide if some keywords that still work in the "defaults" section should be forbidden.

- default_backend : in the matrix, "defaults" was not supported but the keyword details say it is.
Tests also shows it works, then I've updated the matrix.

- capture cookie : in the keyword details, we can read `It is not possible to specify a capture in a "defaults" section.'.
Ok, even if the tests worked, I've added an alert in the configuration parser (as it is for capture request/response header).

- description : not supported in "defaults", I added an alert in the parser.
I've also noticed that this keyword doesn't appear in the documentation.
There's one "description" entry, but for the "global" section, which is for a different use (the patch doesn't update the documentation).

- grace : even if this is maybe useless, it works in "defaults". Documentation is updated.
- redirect : alert is added in the parser.
- rsprep : alert added in the parser.

--
Cyril Bonté