…t, javiereguiluz)

This PR was submitted for the 3.2 branch but it was merged into the 2.7 branch instead (closes #7664).

Discussion
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Add an explanation about «constraints» validation

Added an explanation about the use of «constraints» in Form Classes and the requirement of «ValidatorExtension» if «constraints» keyword is used.

This is currently not explained in doc, nor in Forms, Validation, Custom validation or any other page.

Commits
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c9f1b95 Rewords and minor fixes
cb6b2e6 Add an explanation about «constraints» validation

* 2.7: (31 commits)
  Fixed the RST syntax
  Improve example context
  [#5621] Enhancing example of using bundle config
  [#7601] minor tweak
  Update expiration.rst
  Update expiration.rst
  Update expiration.rst
  Update expiration.rst
  Minor reword and fixed the line length
  Improve specification explanation
  [#7664] minor wording tweak
  Rewords and minor fixes
  Add an explanation about «constraints» validation
  [#7645] enumerate ordered list items implicitly
  Adding a new article about "Creating a Bug Reproducer"
  Fixed a syntax issue
  Use backticks
  #7311 choice_value callback argument can be null
  Fixed broken links for nginx & FastCgiExternalServer
  Update dialoghelper.rst
  ...

* 2.8: (46 commits)
  [#7507] fix component name
  [#7490] minor typo fix
  Added a note about redirections to absolute URLs in tests
  Added the changes suggested by reviewers
  [#7620] use generate() in PHP templates before 2.8
  Fixed the RST syntax
  Improve example context
  [#5621] Enhancing example of using bundle config
  [#7601] minor tweak
  Update expiration.rst
  Update expiration.rst
  Update expiration.rst
  Update expiration.rst
  Minor reword and fixed the line length
  Improve specification explanation
  [#7664] minor wording tweak
  Rewords and minor fixes
  Add an explanation about «constraints» validation
  [#7645] enumerate ordered list items implicitly
  Adding a new article about "Creating a Bug Reproducer"
  ...