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).

Add comments, update the manual.

n  - big endian int16
N  - big endian int32
qQ - big endian int64
f  - float
d  - double