doc: Add deps install notes for multiprocess

These just mirror the content in src/ipc/libmultiprocess/doc/install.md

Author: TheCharlatan

doc/build-osx.md

@@ -125,6 +125,19 @@ For more information on ZMQ, see: [zmq.md](zmq.md)
 
 ---
 
+### IPC Dependencies
+
+Compiling IPC-enabled binaries with `-DENABLE_IPC=ON` requires the following dependency.
+Skip if you do not need IPC functionality.
+
+```bash
+brew install capnp
+```
+
+For more information on IPC, see: [multiprocess.md](multiprocess.md).
+
+---
+
 #### Test Suite Dependencies
 
 There is an included test suite that is useful for testing code changes when developing.

doc/build-unix.md

@@ -68,6 +68,11 @@ User-Space, Statically Defined Tracing (USDT) dependencies:
 
     sudo apt install systemtap-sdt-dev
 
+IPC-enabled binaries are compiled  with `-DENABLE_IPC=ON` and require the following dependencies.
+Skip if you do not need IPC functionality.
+
+    sudo apt-get install libcapnp-dev capnproto
+
 GUI dependencies:
 
 Bitcoin Core includes a GUI built with the cross-platform Qt Framework. To compile the GUI, we need to install
@@ -118,6 +123,11 @@ User-Space, Statically Defined Tracing (USDT) dependencies:
 
     sudo dnf install systemtap-sdt-devel
 
+IPC-enabled binaries are compiled  with `-DENABLE_IPC=ON` and require the following dependency.
+Skip if you do not need IPC functionality.
+
+    sudo dnf install capnproto
+
 GUI dependencies:
 
 Bitcoin Core includes a GUI built with the cross-platform Qt Framework. To compile the GUI, we need to install