Fix broken xmllint validation.

An older version of xmllint failed to validate a broken
URL.

The URL led to nowhere, so this also fixes a few dangling
links.

Use olink & targetptr instead of link & linkend, as is the
convention in the rest of the manual.

Give internal links a bit longer names (as is also convention
in the rest of the manual).
Merge pull request #92 from tarantool-bot/stable

Fix broken xmllint validation.
Merge branch 'stable' of https://github.com/tarantool/tarantool into stable
stored-procedures.xml update