Docs fix ref and title

[ilyar]

Pretty Ref and Fixed WARNING: Duplicate explicit target name

/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".

Fixed WARNING: Title underline too short

/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.

[doctrinebot]

Hello,

thank you for creating this pull request. I have automatically opened an issue
on our Jira Bug Tracker for you. See the issue link:

http://www.doctrine-project.org/jira/browse/DDC-3846

We use Jira to track the state of pull requests and the versions they got
included in.

[Ocramius]

@ilyar thanks!