tcltest::bytestring is deprecated. Document that (and don't use it an… #4584

Job	Run time
msvc	10m 30s
gcc	11m 56s
msvc (OPTS=static,msvcrt)	9m 41s
gcc (--disable-shared)	12m 10s
gcc (--enable-symbols)	11m 58s
gcc (--enable-symbols=mem)	12m 50s
msvc (OPTS=static,staticpkg,msvcrt)	11m 5s
gcc (--enable-symbols=all)	12m 44s
msvc (OPTS=symbols)	9m 21s
msvc (OPTS=symbols STATS=compdbg,memdbg)	11m 24s
	1h 53m 39s

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