feat(doc): add more information on Fastly tag keys

[94noni]

Hey, here is a PR after #588
Fastly needs a separator so it could be wise to state it here :) (instead of default ,)

https://www.fastly.com/documentation/reference/http/http-headers/Surrogate-Key/

wdyt? thank you,

[dbu]

Suggested change

	Configure the tag header to ``Surrogate-Key``. (``fos_http_cache.tags.response_header`` as well as ``fos_http_cache.tags.separator`` to `` `` if you use FOSHttpCacheBundle)
	Configure the tag header to ``Surrogate-Key`` and the separator to `` `` (empty string).
	If you use FOSHttpCacheBundle, this can be done in the bundle configuration:
	.. code-block:: yaml
	fos_http_cache:
	tags:
	response_header: Surrogate-Key
	separator: ' '
	If you do not use the bundle, you need to pass your own instance of the TagHeaderFormatter with the right options to the ResponseTagger::
	use FOS\HttpCache\ResponseTagger;
	use FOS\HttpCache\TagHeaderFormatter\CommaSeparatedTagHeaderFormatter;
	// ...
	$tagger = new ResponseTagger([
	'header_formatter' => new CommaSeparatedTagHeaderFormatter('Surrogate-Key', ' '),
	]);

[dbu]

thanks a lot! i never used fastly myself, maybe the person contributing this did not remember to document it.

i suggest we make this a bit more verbose while we are at it, to provide the full information.

[94noni]

I will, let me come back to this PR soon
in the mean time, may I propose this PR in the bundle FriendsOfSymfony/FOSHttpCacheBundle#638

if accepted, I will rewrite this PR to document both config when using the sf bundle

[94noni]

closing, I will try to open a PR to document this separator as well as the new bundle config of ttl header name

[dbu]

we could have merged this and do a separate PR for the ttl header name... but now that its closed you can also do it all in one PR :-)