Improve canvas layer index documentation #259

Job	Run time
📊 Static checks / Code style, file formatting, and docs	3m 28s
🐧 Linux / Editor w/ Mono (target=editor)	1h 12m 41s
🐧 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)	58m 26s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	40m 53s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	34m 1s
🤖 Android / Template (target=template_release)	1h 0m 51s
🍏 iOS / Template (target=template_release)	36m 4s
🍎 macOS / Editor (target=editor, tests=yes)	42m 47s
🐧 Linux / Template w/ Mono (target=template_release)	32m 23s
🍎 macOS / Template (target=template_release)	37m 42s
🐧 Linux / Minimal template (target=template_release, everything disabled)	19m 24s
🌐 Web / Template (target=template_release)	32m 15s
🏁 Windows / Editor (target=editor, tests=yes)	1h 6m 32s
🏁 Windows / Template (target=template_release)	45m 33s
🪲 Godot CPP / Build and test Godot CPP	9m 38s
	9h 52m 38s

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