Fix documentation about automatic behavior of SYMFONY_HTTP_CACHE

[peterkeung]

I can see no code that conditionally sets SYMFONY_HTTP_CACHE based on SYMFONY_TRUSTED_PROXIES

Also the extra phrase about it being empty makes it more confusing than needed. The phrase about the default behavior is good enough.

[benkmugo]

+1
I think the SYMFONY_HTTP_CACHE default behaviour line could be made clearer though

e.g.
if empty + prod = enabled
if not set + prod = enabled
if set explictly 0|1 = disabled|enabled regardless of env

[andrerom]

Well the doc here was trying to explain this in app.php:

// Depending on the SYMFONY_HTTP_CACHE environment variable, tells whether the internal HTTP Cache mechanism is to be used.
// Recommendation is to use Varnish over this, for performance and being able to setup cluster if you need to.
// If not set, or "", it is auto activated if _not_ in "dev" environment.
if (($useHttpCache = getenv('SYMFONY_HTTP_CACHE')) === false || $useHttpCache === '') {
    $useHttpCache = $environment !== 'dev';
}

[peterkeung]

In reviewing what you just pasted, I think the suggested doc edit makes sense.