Document precise GC

[rainers]

Documentation for dlang/druntime#2418

[dlang-bot]

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[thewilsonator]

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./spec/garbage.dd:452:		be interpreted as a reference to other memory as a "false pointer". The collector
./spec/garbage.dd:453:		has to make pessimistic assumptions if a memory slot can contain both a pointer or
./spec/garbage.dd:461:			struct S { size_t hash; Data* data; }
posix.mak:822: recipe for target 'test' failed