minor #19731 [Yaml] improve changelog and upgrade entries (xabbuh)

This PR was merged into the 3.2-dev branch.

Discussion
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[Yaml] improve changelog and upgrade entries

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  |no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Improves the wording as suggested by @javiereguiluz in symfony/symfony#19538 (comment).

Commits
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e288256 [Yaml] improve changelog and upgrade entries

Author: fabpot

UPGRADE-3.2.md

@@ -77,5 +77,6 @@ Yaml
  * Support for silently ignoring duplicate keys in YAML has been deprecated and
    will lead to a `ParseException` in Symfony 4.0.
 
- * Mappings with a colon that is not followed by a space are deprecated and
-   will lead to a `ParseException` in Symfony 4.0.
+ * Mappings with a colon (`:`) that is not followed by a whitespace are deprecated
+   and will lead to a `ParseException` in Symfony 4.0 (e.g. `foo:bar` must be
+   `foo: bar`).

UPGRADE-4.0.md

@@ -147,8 +147,9 @@ Translation
 Yaml
 ----
 
- * Mappings with a colon that is not followed by a space are are not
-   supported anymore and lead to a `ParseException`.
+ * Mappings with a colon (`:`) that is not followed by a whitespace are not
+   supported anymore and lead to a `ParseException`(e.g. `foo:bar` must be
+   `foo: bar`).
 
  * Starting an unquoted string with `%` leads to a `ParseException`.
 

src/Symfony/Component/Yaml/CHANGELOG.md

@@ -4,8 +4,9 @@ CHANGELOG
 3.2.0
 -----
 
- * Mappings with a colon that is not followed by a space are deprecated and
-   will lead to a `ParseException` in Symfony 4.0.
+ * Mappings with a colon (`:`) that is not followed by a whitespace are deprecated
+   and will lead to a `ParseException` in Symfony 4.0 (e.g. `foo:bar` must be
+   `foo: bar`).
 
  * Added support for parsing PHP constants: