Fix docstrings of various `lu` functions #3056

Job	Run time
GCC build regression check (no tests)	23s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 13s
macOS-M1, Clang (x3)	7m 58s
MinGW GCC (x0.5)	10m 5s
Nemo.jl	18m 36s
MSVC (x1)	9m 30s
Ubuntu Clang, BLAS and examples (x5)	12m 34s
Ubuntu GCC, Code Coverage (x10)	29m 39s
Ubuntu GCC with NTL (assert, x2)	9m 24s
	1h 52m 22s

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