ci: remove -Wdocumentation from -Werror in multiprocess CI

The issues here are coming from Boost Test code.

ci/test/00_setup_env_i686_multiprocess.sh

@@ -14,5 +14,5 @@ export DEP_OPTS="DEBUG=1 MULTIPROCESS=1"
 export GOAL="install"
 export TEST_RUNNER_EXTRA="--v2transport"
 export BITCOIN_CONFIG="--enable-debug CC='clang -m32' CXX='clang++ -m32' \
-CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE'"
+CPPFLAGS='-DBOOST_MULTI_INDEX_ENABLE_SAFE_MODE' CXXFLAGS='-Wno-error=documentation'"
 export BITCOIND=bitcoin-node  # Used in functional tests