doc: Document WITH_EXTERNAL_LIBMULTIPROCESS build option better

This includes a cmake documentation change suggested
bitcoin/bitcoin#31741 (comment)
and another change to mention the option in markdown documentation

Co-authored-by: Cory Fields <cory-nospam-@coryfields.com>

Author: ryanofsky

CMakeLists.txt

@@ -154,7 +154,7 @@ endif()
 cmake_dependent_option(WITH_DBUS "Enable DBus support." ON "CMAKE_SYSTEM_NAME STREQUAL \"Linux\" AND BUILD_GUI" OFF)
 
 option(ENABLE_IPC "Build multiprocess bitcoin-node and bitcoin-gui executables in addition to monolithic bitcoind and bitcoin-qt executables. Requires libmultiprocess library. Experimental." OFF)
-cmake_dependent_option(WITH_EXTERNAL_LIBMULTIPROCESS "Build with external libmultiprocess library instead of with local git subtree when ENABLE_IPC is enabled. This is not normally recommended, but can be useful when cross-compiling or making changes to the upstream project." OFF "ENABLE_IPC" OFF)
+cmake_dependent_option(WITH_EXTERNAL_LIBMULTIPROCESS "Build with external libmultiprocess library instead of with local git subtree when ENABLE_IPC is enabled. This is not normally recommended, but can be useful for developing libmultiprocess itself." OFF "ENABLE_IPC" OFF)
 if(ENABLE_IPC AND WITH_EXTERNAL_LIBMULTIPROCESS)
   find_package(Libmultiprocess REQUIRED COMPONENTS Lib)
   find_package(LibmultiprocessNative REQUIRED COMPONENTS Bin

doc/multiprocess.md

@@ -35,6 +35,10 @@ The `cmake` build will pick up settings and library locations from the depends d
 
 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.
 
+### External libmultiprocess installation
+
+By default when `-DENABLE_IPC=ON` is enabled, the libmultiprocess sources at [../src/ipc/libmultiprocess/](../src/ipc/libmultiprocess/) are built as part of the bitcoin cmake build, but alternately an external [libmultiprocess](https://github.com/bitcoin-core/libmultiprocess/) cmake package can be used instead by following its [installation instructions](https://github.com/bitcoin-core/libmultiprocess/blob/master/doc/install.md) and specifying `-DWITH_EXTERNAL_LIBMULTIPROCESS=ON` to the bitcoin build, so it will use the external package instead of the sources. This can be useful when making changes to the upstream project. If libmultiprocess is not installed in a default system location it is possible to specify the [`CMAKE_PREFIX_PATH`](https://cmake.org/cmake/help/latest/envvar/CMAKE_PREFIX_PATH.html) environment variable to point to the installation prefix where libmultiprocess is installed.
+
 ## Usage
 
 `bitcoin-node` is a drop-in replacement for `bitcoind`, and `bitcoin-gui` is a drop-in replacement for `bitcoin-qt`, and there are no differences in use or external behavior between the new and old executables. But internally after [#10102](https://github.com/bitcoin/bitcoin/pull/10102), `bitcoin-gui` will spawn a `bitcoin-node` process to run P2P and RPC code, communicating with it across a socket pair, and `bitcoin-node` will spawn `bitcoin-wallet` to run wallet code, also communicating over a socket pair. This will let node, wallet, and GUI code run in separate address spaces for better isolation, and allow future improvements like being able to start and stop components independently on different machines and environments.