[WIP][Valiadtor] - EmailConstraint reference
Use of Sphinx markup
Version added and clarifications
Link moved and heading updated
Typo and link label
Lowercase and link label
CS
Blank line restored
Enhanced Firewall Restrictions docs

Squashed commits:
[2081fc4] Enhanced Firewall Restrictions docs for PR symfony/symfony#10404
[8f065bc] Add redirect to new page
[b42e80f] fixed typos
[cc6b07d] tiny update
After backporting something from master (2.5), removing the 2.5-specific features
[#3539] A backport of a sentence - the parts that apply to 2.3
add "Commands as Services" chapter to the cookbook's map
The host parameter has to be in defaults, not requirements
Moved tip block to remove confusion
feature #3368 The host parameter has to be in defaults, not requirements (MarieMinasyan)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3368).

Discussion
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The host parameter has to be in defaults, not requirements

 Q             | A
 ------------- | ---
 Doc fix?      | yes
 New docs?     | no
 Applies to    | 2.2+

Also has to be applied to 2.2 & 2.3 branches

Commits
-------

8a1afae Moved tip block to remove confusion
9c30509 The host parameter has to be in defaults, not requirements
Minor tweaks related to #3368
Update file_uploads.rst

Code examples do not apply to Doctrine EventSubscribers scenario, on preUpdate.
Cheers.
Update file_uploads.rst

Updated as per @RyanWeaver's suggestions.
Rewording Caution paragraph at line 413
Going back to the ``Caution`` alternative.