Clarify `PackedByteArray.decompress*` limitations with external data #583

Job	Run time
📊 Static checks / Code style, file formatting, and docs	2m 31s
🍎 macOS / Editor (target=editor, tests=yes)	5m 39s
🤖 Android / Template (target=template_release)	4m 40s
🍏 iOS / Template (target=template_release)	3m 27s
🐧 Linux / Editor w/ Mono (target=editor)	5m 46s
🍎 macOS / Template (target=template_release)	3m 22s
🐧 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)	18m 25s
🐧 Linux / Editor with clang sanitizers (target=editor, tests=yes, dev_build=yes, use_asan=yes, use_ubsan=yes, use_llvm=yes, linker=lld)	6m 55s
🌐 Web / Template (target=template_release)	1m 52s
🏁 Windows / Editor (target=editor, tests=yes)	8m 35s
🐧 Linux / Template w/ Mono (target=template_release)	1m 51s
🏁 Windows / Template (target=template_release)	6m 35s
🐧 Linux / Minimal template (target=template_release, everything disabled)	1m 31s
🪲 Godot CPP / Build and test Godot CPP	6m 43s
	1h 17m 52s

Provide feedback