[NFC][ntuple] Add `AddProjectedField` documentation #12901

Job	Run time
mac13 ARM64 LLVM_ENABLE_ASSERTIONS=On, builtin_zlib=ON	29m 3s
mac14 X64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	1h 28m 15s
mac15 ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	36m 3s
mac-beta ARM64 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	36m 42s
Windows 10 x64 Release	4h 11m 23s
Windows 10 x86 Release	1h 9m 32s
Upload Event Payload	2s
fedora39 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	2h 0m 59s
alma8 LLVM_ENABLE_ASSERTIONS=On	1h 21m 51s
alma9 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	2h 5m 51s
ubuntu20 LLVM_ENABLE_ASSERTIONS=On	1h 30m 14s
ubuntu22 imt=Off, LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	2h 3m 53s
ubuntu2404 LLVM_ENABLE_ASSERTIONS=On, CMAKE_BUILD_TYPE=Debug	2h 18m 39s
debian125 LLVM_ENABLE_ASSERTIONS=On, CMAKE_CXX_STANDARD=20	1h 3m 1s
alma9 modules_off runtime_cxxmodules=Off	52m 9s
alma9 march_native CMAKE_BUILD_TYPE=RelWithDebInfo, CMAKE_CXX_FLAGS=-march=native, CMAKE_C_FLAGS=-march=native, fortran=OFF	1h 18m 49s
alma9 arm64 CMAKE_BUILD_TYPE=RelWithDebInfo, ccache=ON	49m 13s
alma9-clang clang LLVM_ENABLE_ASSERTIONS=On, CMAKE_C_COMPILER=clang, CMAKE_CXX_COMPILER=clang++	1h 9m 31s
ubuntu2404-cuda gpu	1m 21s
	1d 1h 6m 31s

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