Clarify documentation

Change-Id: I7066c26c5af8233da2c024ca8835f0e389c07ecb

ci/travis_before_script_cpp.sh

@@ -144,7 +144,8 @@ else
 fi
 
 # Build and install libraries. Configure ARROW_CPP_BUILD_TARGETS environment
-# variable to only build certain targets
+# variable to only build certain targets. If you use this, you must also set
+# the environment variable ARROW_TRAVIS_OPTIONAL_INSTALL=1
 $TRAVIS_MAKE -j4 $ARROW_CPP_BUILD_TARGETS
 $TRAVIS_MAKE install
 

cpp/CMakeLists.txt

@@ -99,7 +99,7 @@ if("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_CURRENT_SOURCE_DIR}")
     "Compiler flags to append when compiling Arrow")
 
   option(ARROW_OPTIONAL_INSTALL
-    "If enabled install targets that have already been built"
+    "If enabled install ONLY targets that have already been built"
     OFF)
 
   option(ARROW_ONLY_LINT