Update doc dql-custom-walkers.rst with an output walker to interpolate parameters into SQL

[n0099]

#11959

[greg0ire]

Sorry for inadvertently closing your other PR yesterday, it was because I deleted the target branch.

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/3.5.x, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already pushed. Don't forget the --force option otherwise git will try to merge both things together.

[greg0ire]

Ok, then what? You should probably state the use case for this explicitly.

[n0099]

This example would help https://stackoverflow.com/questions/2095394/doctrine-how-to-print-out-the-real-sql-not-just-the-prepared-statement and https://stackoverflow.com/questions/4155288/how-to-write-union-in-doctrine-2-0/79647475#79647475

[greg0ire]

You should probably include a summary of this in this doc you are contributing.