doc: Update documentation to explain libmultiprocess subtree

Co-authored-by: Cory Fields <[email protected]>

Author: ryanofsky

doc/developer-notes.md

@@ -1294,6 +1294,9 @@ Current subtrees include:
 - src/minisketch
   - Upstream at https://github.com/bitcoin-core/minisketch ; maintained by Core contributors.
 
+- src/ipc/libmultiprocess
+  - Upstream at https://github.com/bitcoin-core/libmultiprocess ; maintained by Core contributors.
+
 Upgrading LevelDB
 ---------------------
 

doc/multiprocess.md

@@ -12,7 +12,11 @@ The `-debug=ipc` command line option can be used to see requests and responses b
 
 ## Installation
 
-The multiprocess feature requires [Cap'n Proto](https://capnproto.org/) and [libmultiprocess](https://github.com/bitcoin-core/libmultiprocess) as dependencies. A simple way to get started using it without installing these dependencies manually is to use the [depends system](../depends) with the `MULTIPROCESS=1` [dependency option](../depends#dependency-options) passed to make:
+Specifying `-DENABLE_IPC=ON` requires [Cap'n Proto](https://capnproto.org/) to be installed. See [build-unix.md](build-unix.md) and [build-osx.md](build-osx.md) for information about installing dependencies.
+
+### Depends installation
+
+Alternately the [depends system](../depends) can be used to avoid need to install local dependencies. A simple way to get started is to pass the `MULTIPROCESS=1` [dependency option](../depends#dependency-options) to make:
 
 ```
 cd <BITCOIN_SOURCE_DIRECTORY>
@@ -27,7 +31,9 @@ BITCOIND=$(pwd)/build/bin/bitcoin-node build/test/functional/test_runner.py
 
 The `cmake` build will pick up settings and library locations from the depends directory, so there is no need to pass `-DENABLE_IPC=ON` as a separate flag when using the depends system (it's controlled by the `MULTIPROCESS=1` option).
 
-Alternately, you can install [Cap'n Proto](https://capnproto.org/) and [libmultiprocess](https://github.com/bitcoin-core/libmultiprocess) packages on your system, and just run `cmake -B build -DENABLE_IPC=ON` without using the depends system. The `cmake` build will be able to locate the installed packages via [pkg-config](https://www.freedesktop.org/wiki/Software/pkg-config/). See [Installation](https://github.com/bitcoin-core/libmultiprocess/blob/master/doc/install.md) section of the libmultiprocess readme for install steps. See [build-unix.md](build-unix.md) and [build-osx.md](build-osx.md) for information about installing dependencies in general.
+### Cross-compiling
+
+When cross-compiling and not using depends, native code generation tools from [libmultiprocess](https://github.com/bitcoin-core/libmultiprocess) and [Cap'n Proto](https://capnproto.org/) are required. They can be passed to the cmake build by specifying `-DMPGEN_EXECUTABLE=/path/to/mpgen -DCAPNP_EXECUTABLE=/path/to/capnp -DCAPNPC_CXX_EXECUTABLE=/path/to/capnpc-c++` options.
 
 ## Usage