Add docs for the new VarDumper features

[javiereguiluz]

I don't know if all of them should be documented, but VarDumper added a ton of new features in Symfony 3.2:

• Add maxDepth & maxStringLength display options ([VarDumper] Add maxDepth & maxStringLength display options symfony#18948), will be covered by Document new environment variables and display options #7217
• Get dump as string with $dumper->dump(..., true); ([VarDumper] Get dump as string with $dumper->dump(..., true); symfony#19755)
• Add LinkStub to create links in HTML dumps ([VarDumper] Add LinkStub to create links in HTML dumps symfony#19816)
• Add ClassStub for clickable & shorter PHP identifiers ([VarDumper] Add ClassStub for clickable & shorter PHP identifiers symfony#19826)
• Add flags to allow fine tuning dumps representation ([VarDumper] Add flags to allow fine tuning dumps representation symfony#17305), will be covered by Document new environment variables and display options #7217
• Allow dumping subparts of cloned Data structures ([VarDumper] Allow dumping subparts of cloned Data structures symfony#19672)
• Optionally pass dumper into DumpExtension ([Bridge][Twig] Optionally pass dumper into DumpExtension symfony#18466)

[jokaio]

symfony/symfony#19755 has already been documented with this commit

[jokaio]

symfony/symfony#19672 has already been documented with this commit

[sawmurai]

No idea where / if to document symfony/symfony#18466. Any ideas anyone?

This PR ads the missing documentation (#18948 and #17305). The rest (except for #18466) already is documented.

PR: #7217

[xabbuh]

@phrix32 Thank you for the great investigation. I have transformed the issue description into a list of tasks and checked both items.

[sawmurai]

symfony/symfony#19816 and symfony/symfony#19826 were documented with 3033bc7.

symfony/symfony#18948 and symfony/symfony#17305 are documented with #7217. So only symfony/symfony#18466 is left. No idea where do document this, tho.

[xabbuh]

Right now you have to replace the var_dumper.html_dumper service or modify it to your needs in a compiler pass when using the full stack Symfony framework.

[sawmurai]

@xabbuh Should that be documented at all? I would think that most of the time the default HtmlDumper is used anyway. If it should be documented: how? Provide config example for a service definition or simply class instantiation?

[xabbuh]

I am not sure if it's really worth documenting that.

[chalasr]

I think the HtmlDumper search utility added in 3.3 deserves a mention as well (symfony/symfony#21109, symfony/symfony#21238)

[xabbuh]

@chalasr thanks, I created #7381