Improve API docs for add_dll_directory #1684

Job	Run time
build-installer (3.4.1, x86-msvcrt, 3.1.6/x64, v3_4_1)	15m 14s
build-installer (3.4.1, x64-ucrt, 3.1.6/x64, v3_4_1)	19m 42s
build-installer (3.3.6, x86-msvcrt, 3.1.6/x64, v3_3_6)	14m 57s
build-installer (3.3.6, x64-ucrt, 3.1.6/x64, v3_3_6)	18m 35s
build-installer (3.2.6, x86-msvcrt, 3.1.6/x64, v3_2_6)	16m 21s
build-installer (3.2.6, x64-ucrt, 3.1.6/x64, v3_2_6)	16m 46s
build-installer (3.1.6, x86-msvcrt, 3.0.7/x64, v3_1_6)	16m 18s
build-installer (3.1.6, x64-ucrt, 3.0.7/x64, v3_1_6)	17m 5s
build-installer (head, x64-ucrt, 3.0.7/x64, master)	3m 41s
build-installer (head, x86-msvcrt, 3.0.7/x64, master)	3m 8s
	2h 21m 47s

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