DDC-3846: [GH-1473] Docs fix ref and title

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of ilyar:

Url: #1473

Message:

Pretty Ref and Fixed WARNING: Duplicate explicit target name

{quote}
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
/doctrine2/docs/en/changelog/migration_2_5.rst:2: WARNING: Duplicate explicit target name: "pull request".
{quote}

Fixed WARNING: Title underline too short

{quote}
/doctrine2/docs/en/reference/second-level-cache.rst:622: WARNING: Title underline too short.
/doctrine2/docs/en/reference/caching.rst:89: WARNING: Title underline too short.
{quote}

[doctrinebot]

Issue was closed with resolution "Fixed"