feature #4749 [2.3][Book][Security] Add isPasswordValid doc as in 2.6 (xelaris)

This PR was merged into the 2.3 branch.

Discussion
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[2.3][Book][Security] Add isPasswordValid doc as in 2.6

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | >=2.3
| Fixed tickets |

As mentioned in #4744 (comment) the paragraph from 2.6 branch is also applicable to 2.3.

Commits
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0f67a1d [Book][Security] Add isPasswordValid doc as in 2.6

Author: weaverryan

Diff for: book/security.rst

@@ -1117,6 +1117,10 @@ In order for this to work, just make sure that you have the encoder for your
 user class (e.g. ``AppBundle\Entity\User``) configured under the ``encoders``
 key in ``app/config/security.yml``.
 
+The ``$encoder`` object also has an ``isPasswordValid`` method, which takes
+the ``User`` object as the first argument and the plain password to check
+as the second argument.
+
 .. caution::
 
     When you allow a user to submit a plaintext password (e.g. registration