New material:
* Add documentation for `filterload` message to devref. This is the last
P2P protocol message which needed documentation.
* Add an example for creating a bloom filter to the devex, as well as an
example of checking data against that filter.
Edits:
* Change "object" to "element" in previous `filteradd` text. I decided
"transaction element" made more sense than the more generic "object".
Text should be fully consistent across both `filterload` and
`filteradd` descriptions.
* Mentioned that I think the example hexdump in the `alert` section is
public domain. (Only the hex is taken from the wiki; the annotation is
my own work.)
