doc: update build and dependencies docs for IPC

OpenBSD does not have this package, so recommend building from
source for now.

doc/build-freebsd.md

@@ -21,6 +21,14 @@ pkg install sqlite3
 
 To build Bitcoin Core without the wallet, use `-DENABLE_WALLET=OFF`.
 
+Cap'n Proto is needed for IPC functionality (see [multiprocess.md](multiprocess.md)):
+
+```bash
+pkg install capnproto
+```
+
+Compile with `-DENABLE_IPC=OFF` if you do not need IPC functionality.
+
 See [dependencies.md](dependencies.md) for a complete overview.
 
 ### 2. Clone Bitcoin Repo