Added a note about the lacking features of Yaml Component

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

In `symfony/symfony` repository there are a lot of issues reporting errors of the Yaml Component. These aren't errors but lacking features of the Yaml Component.

This PR tries to clearly state that Yaml Component lacks lots of features on purpose and provides a simple list of the most notable missing features.
I think multi-line quoted messages and compact block collections could be fixed easily. I'll have a look at both issues. For the rest, I think we can improve the user experience a lot if we provide more sensible exception messages like the one I mentioned in symfony/symfony#8076.