MINOR: [C++] Fix C++17-related comments (#48598)

### Rationale for this change

As a followup to #48414, fix the comments that I had forgotten to update.

### Are these changes tested?

No testing required.

### Are there any user-facing changes?

No.

Authored-by: Antoine Pitrou <[email protected]>
Signed-off-by: Nic Crane <[email protected]>

Author: pitrou

cpp/cmake_modules/SetupCxxFlags.cmake

@@ -139,7 +139,7 @@ if(NOT DEFINED CMAKE_C_STANDARD)
   set(CMAKE_C_STANDARD 11)
 endif()
 
-# This ensures that things like c++17 get passed correctly
+# This ensures that a standard higher than the minimum can be passed correctly
 if(NOT DEFINED CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 20)
 elseif(${CMAKE_CXX_STANDARD} VERSION_LESS 20)

cpp/src/arrow/CMakeLists.txt

@@ -1180,7 +1180,7 @@ endif()
 
 foreach(LIB_TARGET ${ARROW_LIBRARIES})
   target_compile_definitions(${LIB_TARGET} PRIVATE ARROW_EXPORTING)
-  # C++17 is required to compile against Arrow C++ headers and libraries
+  # C++20 is required to compile against Arrow C++ headers and libraries
   target_compile_features(${LIB_TARGET} PUBLIC cxx_std_20)
 endforeach()
 

matlab/README.md

@@ -64,7 +64,7 @@ To build the MATLAB Interface to Apache Arrow from source, the following softwar
 
 1. [MATLAB](https://www.mathworks.com/products/get-matlab.html)
 2. [CMake](https://cmake.org/cmake/help/latest/)
-3. C++ compiler which supports C++17 (e.g. [`gcc`](https://gcc.gnu.org/) on Linux, [`Xcode`](https://developer.apple.com/xcode/) on macOS, or [`Visual Studio`](https://visualstudio.microsoft.com/) on Windows)
+3. C++ compiler which supports C++20 (e.g. [`gcc`](https://gcc.gnu.org/) on Linux, [`Xcode`](https://developer.apple.com/xcode/) on macOS, or [`Visual Studio`](https://visualstudio.microsoft.com/) on Windows)
 4. [Git](https://git-scm.com/)
 
 ## Setup

matlab/tools/cmake/BuildMatlabArrowInterface.cmake

@@ -130,7 +130,7 @@ libmexclass_client_add_proxy_library(
     INCLUDE_DIRS ${MATLAB_ARROW_LIBMEXCLASS_CLIENT_PROXY_LIBRARY_INCLUDE_DIRS}
     LINK_LIBRARIES arrow_shared
 )
-# Use C++17
+# Use C++20
 target_compile_features(${MATLAB_ARROW_LIBMEXCLASS_CLIENT_PROXY_LIBRARY_NAME} PRIVATE cxx_std_20)
 target_compile_definitions(${MATLAB_ARROW_LIBMEXCLASS_CLIENT_PROXY_LIBRARY_NAME} PRIVATE ARROW_MATLAB_EXPORTING)