improve the serialization of custom user models

(cherry picked from commit 7b34eabe4b506c3e49e4dedbefee66c1612bb83d)
[#3307] Adding an additional note about deserialization issues

(cherry picked from commit 7325ec5044c0339275a9700e767d88707116f79d)
[#3307] Fix thanks to @xabbuh

(cherry picked from commit 38612bc190ab2f97f2fbd3f3bc18a591df153cbe)
Clarifying some details on serialize/unserialize and making it consistent with changes we recently made

(cherry picked from commit f285c5a572ee88e94a9314e152a5075243811d49)

Conflicts:
	cookbook/security/entity_provider.rst
Changed sha1 into bcrypt
[#3356] Fixing ticks that don't work inside a bold and removing extra details in intro area that aren't necessary (you'll need to read the cookbook entry for more details anyways)
[#3356] Changing to use the bcrypt algorithm

Also removing one example (now possible with using bcrypt from the beginning) and related tweaks.
[#3356] Moving serialize and Equatable logic further down for clarity
[#3356] Clarifying when you need a salt

Also filling in other details related to using BCrypt
[#3405] Tweaks thanks to @WouterJ and @xabbuh
Merge pull request #3405 from symfony/security_fix_extended

Changed sha1 into bcrypt continued...
Fixed YAML code blocks in the book
Fixed (mostly) yaml code blocks

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats
some language tweaks (AE, third-person perspective)
Merge branch '2.2' into 2.3

Conflicts:
	book/security.rst
	cookbook/security/entity_provider.rst
minor #3497 Fix highlighting (WouterJ)

This PR was merged into the 2.3 branch.

Discussion
----------

Fix highlighting

Initial idea was to fix the highlighting. While doing that, others things
also got a fix:

* indenting
* missing formats
* some `...` in XML elements
* missing comments in other formats

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | partially https://github.com/symfony/symfony-docs/issues/1886

Commits
-------

7c0cf89 Fixed (mostly) yaml code blocks
2a43a97 Fixed YAML code blocks in the book
minor #3451 some language tweaks (AE, third-person perspective) (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

some language tweaks (AE, third-person perspective)

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

0aad5d3 some language tweaks (AE, third-person perspective)