Clarify `UndoRedo` `MergeMode` and `add_*_reference` docs #26854

Job	Run time
📊 Static checks / Code style, file formatting, and docs	3m 28s
🐧 Linux / Editor w/ Mono (target=editor)	4m 43s
🐧 Linux / Editor with doubles and GCC sanitizers (target=editor, tests=yes, dev_build=yes, scu_build=yes, precision=double, use_asan=yes, use_ubsan=yes, linker=gold)	16m 42s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	7m 9s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	8m 9s
🍎 macOS / Editor (target=editor, tests=yes)	7m 2s
🐧 Linux / Template w/ Mono (target=template_release)	1m 48s
🍎 macOS / Template (target=template_release)	3m 27s
🐧 Linux / Minimal template (target=template_release, everything disabled)	41s
🤖 Android / Template (target=template_release)	4m 18s
🍏 iOS / Template (target=template_release)	2m 19s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	4m 16s
🏁 Windows / Editor (target=editor, tests=yes)	9m 55s
🌐 Web / Template w/o threads (target=template_release, threads=no)	4m 15s
🏁 Windows / Template (target=template_release)	8m 7s
🪲 Godot CPP / Build and test Godot CPP	7m 0s
	1h 33m 19s

Provide feedback