Improve docs for `slog::Value` trait #95

Job	Run time
test (stable minus 15 releases)	47s
test (stable minus 15 releases, nested-values)	57s
test (stable minus 15 releases, dynamic-keys)	46s
test (stable minus 15 releases, nothreads)	46s
test (stable minus 15 releases, nested-values dynamic-keys)	55s
test (stable minus 15 releases, nested-values dynamic-keys nothreads)	52s
test (1.56)	47s
test (1.56, nested-values)	55s
test (1.56, dynamic-keys)	44s
test (1.56, nothreads)	47s
test (1.56, nested-values dynamic-keys)	58s
test (1.56, nested-values dynamic-keys nothreads)	55s
test (1.81)	13s
test (1.81, nested-values)	20s
test (1.81, dynamic-keys)	16s
test (1.81, nothreads)	14s
test (1.81, nested-values dynamic-keys)	18s
test (1.81, nested-values dynamic-keys nothreads)	18s
test (stable)	10s
test (stable, nested-values)	13s
test (stable, dynamic-keys)	9s
test (stable, nothreads)	6s
test (stable, nested-values dynamic-keys)	14s
test (stable, nested-values dynamic-keys nothreads)	20s
test (nightly)	14s
test (nightly, nested-values)	20s
test (nightly, dynamic-keys)	13s
test (nightly, nothreads)	12s
test (nightly, nested-values dynamic-keys)	18s
test (nightly, nested-values dynamic-keys nothreads)	18s
test (1.49)	45s
test (1.49, dynamic-keys)	44s
test (1.49, nothreads)	47s
test (1.49, nothreads dynamic-keys)	47s
	17m 38s

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