Better documentation for `SYMFONY_DEPRECATIONS_HELPER`

[OskarStark]

We use the following for a slide, maybe someone could add these information to the docs itself:

cc @alexandre-daubois @alamirault @MrYamous

Anyone? 😄

[javiereguiluz]

I downloaded the image and used macOS feature to "copy text from image" to extract the contents:

## Configure Deprecation Behavior

'SYMFONY_DEPRECATIONS_HELPER' environment variable possible values:

'max[total]=n'    | Tests fail when a total of n depreactions is reached
'max[self]=n'     | "self" deprecations are triggered by your own code
'max[direct]=n'   | "direct" deprecations are triggered by vendors used by your code
'max[indirect]=n' | "indirect" deprecations are triggered by vendors that your code doesn't use directly
'disabled'        | Deprecations are not displayed and can't make tests fail
'verbose=0'       | Only display deprecations count
'/regex/'         | For debugging purpose, finds the first deprecation matching regex and displays its stack trace

<div class="text-xs" v-click>
Values can be combined: 'SYMFONY_DEPRECATIONS_HELPER="max[total]=50&max[direct]=10&verbose=0"