Reorder entity loading in manual.xml.in

We disabled entity loading by default on HHVM. That's good. But it caused the
HHVM doc generator to fail when validating, even with the proper whitelists set.

```
:
:
Iterating over extension specific version files... OK
Saving it... OK
Creating file /data/users/joelm/hack-hhvm-docs/__docs/phpdoc/doc-base/entities/file-entities.ent... done
Checking for if we should generate a simplified file... no
Checking whether to save an invalid .manual.xml... no
Loading and parsing manual.xml... failed.
 failed to load external entity "../entities/global.ent"
 internal error

Previous errors too severe. Stopping here.

Eyh man. No worries. Happ shittens. Try again after fixing the errors above.
```

However, reordering the entity statements in `manual.xml.in` seems like it fixes the problem.

```
:
:
Iterating over extension specific version files... OK
Saving it... OK
Creating file /data/users/joelm/hack-hhvm-docs/__docs/phpdoc/doc-base/entities/file-entities.ent... done
Checking for if we should generate a simplified file... no
Checking whether to save an invalid .manual.xml... no
Loading and parsing manual.xml... done.
Validating manual.xml... done.

All good. Saving .manual.xml... done.
All you have to do now is run 'phd -d /data/users/joelm/hack-hhvm-docs/__docs/phpdoc/doc-base/.manual.xml'
If the script hangs here, you can abort with ^C.
                 _ _..._ __
                \)`    (` /
                 /      `\
                |  d  b   |
                =\  Y    =/--..-="````"-.
                  '.=__.-'               `\
                     o/                 /\ \
                      |                 | \ \   / )
                       \    .--""`\    <   \ '-' /
                      //   |      ||    \   '---'
                 jgs ((,,_/      ((,,___/

 (Run `nice php configure.php` next time!)
```

I am wondering if there is a bug in our new call to `xmlCreateIOParserCtxt`, since the call to `xmlCreateFileParserCtxt` worked.

https://github.com/facebook/hhvm/commit/842d6fb7dcd6d669b41bf179ec4602793fa38a7a#diff-9ad3cbaa13a3963b8ceecfa092a320bfL654

Is this a libxml bug, possibly? Maybe it didn't like loading entities in a directory root above the caller and then in a directory rootbelow the caller. Or something absurd like that. Not sure what is going on here.