ICU-12865 Update RelativeDateTimeFormatter API docs for new features in ICU 54... #2826

Job	Run time
clang-valgrind-test	10m 8s
clang-valgrind-intltest (utility)	6m 57s
clang-valgrind-intltest (normalize)	6m 14s
clang-valgrind-intltest (collator)	5m 12s
clang-valgrind-intltest (regex)	5m 45s
clang-valgrind-intltest (format)	16m 30s
clang-valgrind-intltest (translit)	6m 32s
clang-valgrind-intltest (rbbi)	7m 50s
clang-valgrind-intltest (rbnf)	7m 46s
clang-valgrind-intltest (rbnfrt)	6m 26s
clang-valgrind-intltest (icuserv)	5m 45s
clang-valgrind-intltest (idna)	5m 10s
clang-valgrind-intltest (convert)	7m 22s
clang-valgrind-intltest (rbnfp)	5m 22s
clang-valgrind-intltest (csdet)	5m 20s
clang-valgrind-intltest (spoof)	5m 11s
clang-valgrind-intltest (bidi)	6m 2s
	1h 59m 32s

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