Add note to shadow_mesh docs to warn about improper usage #308

Job	Run time
📊 Static checks / Code style, file formatting, and docs	23s
🤖 Android / Editor (target=editor)	4m 54s
🤖 Android / Template arm32 (target=template_release, arch=arm32)	4m 4s
🤖 Android / Template arm64 (target=template_release, arch=arm64)	3m 37s
🐧 Linux / Editor w/ Mono (target=editor)	39m 58s
🐧 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)	47m 51s
🍏 iOS / Template (target=template_release)	1m 49s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	6m 35s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	6m 10s
🐧 Linux / Template w/ Mono (target=template_release)	1m 15s
🍎 macOS / Editor (target=editor, tests=yes)	3m 59s
🐧 Linux / Minimal template (target=template_release, everything disabled)	57s
🌐 Web / Template w/ threads (target=template_release, threads=yes)	6m 38s
🍎 macOS / Template (target=template_release)	3m 2s
🌐 Web / Template w/o threads (target=template_release, threads=no)	6m 24s
🏁 Windows / Editor (target=editor, tests=yes)	51m 57s
🏁 Windows / Template (target=template_release)	37m 4s
🪲 Godot CPP / Build and test Godot CPP	7m 59s
	3h 54m 36s

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