Dev Docs: Describe P2P Messages That Request Or Reply With Data

P2P Reference Preview: http://dg0.dtrt.org/en/developer-reference#p2p-network
P2P Examples Preview: http://dg0.dtrt.org/en/developer-examples#p2p-network

Adds to the devel reference page detailed documentation on the following messages: block, getblocks, getdata, getheaders, headers, inv, mempool, merkleblock, notfound, and tx.

Adds to the devel examples page an example of requesting and parsing a merkleblock message.

Adds to the devel docs overview pages links to the above two new P2P sections.

Tweaks the autocrossref plugin ignore pattern to not crossref in the middle of a GIF image name; this allows the inclusion of animated GIFs.

Note: I'm working on documenting the remaining P2P messages in a separate branch.
Yes, when descending into the right child, you will need to put a flag on the list.  Also, "Never put a flag..." is a correct statement.

However, the current statement looks accurate to me, so I'm not sure that the revised statement is more accurate.  I'm guessing something in the current statement looked confusing to you---could you point that out for me?  (Is it just that the current statement has the subjunctive clause?  "except when...")