Document that `Input.is_action_*` should not be used during input-handling #15426

Job	Run time
📊 Static checks / Code style, file formatting, and docs	40s
🍎 macOS / Editor (target=editor, tests=yes)	4m 11s
🐧 Linux / Editor w/ Mono (target=editor)	5m 59s
🍎 macOS / Template (target=template_release)	4m 47s
🐧 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)	17m 10s
🤖 Android / Template (target=template_release)	3m 10s
🍏 iOS / Template (target=template_release)	3m 5s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	8m 59s
🐧 Linux / Editor with ThreadSanitizer (target=editor, tests=yes, dev_build=yes, use_tsan=yes, use_llvm=yes, linker=lld)	5m 48s
🌐 Web / Template (target=template_release)	1m 40s
🐧 Linux / Template w/ Mono (target=template_release)	2m 3s
🏁 Windows / Editor (target=editor, tests=yes)	10m 5s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 1s
🏁 Windows / Template (target=template_release)	7m 5s
🪲 Godot CPP / Build and test Godot CPP	6m 49s
	1h 22m 32s

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