ICU-23032 Update MeasureUnit documentation for `forIdentifier` method #3431

Job	Run time
clang-valgrind-test	53s
clang-valgrind-intltest (utility)	51s
clang-valgrind-intltest (normalize)	54s
clang-valgrind-intltest (collator)	53s
clang-valgrind-intltest (regex)	49s
clang-valgrind-intltest (format)	52s
clang-valgrind-intltest (translit)	52s
clang-valgrind-intltest (rbbi)	52s
clang-valgrind-intltest (rbnf)	53s
clang-valgrind-intltest (rbnfrt)	52s
clang-valgrind-intltest (icuserv)	52s
clang-valgrind-intltest (idna)	52s
clang-valgrind-intltest (convert)	53s
clang-valgrind-intltest (rbnfp)	54s
clang-valgrind-intltest (csdet)	53s
clang-valgrind-intltest (spoof)	51s
clang-valgrind-intltest (bidi)	54s
	14m 50s

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