doc: Describe interior mutability pattern #47

Job	Run time
build_docs	39s
with_features (ubuntu, 1.77, serde)	53s
with_features (ubuntu, stable, serde)	59s
no_features (ubuntu, 1.77)	52s
with_features (ubuntu, beta, serde)	56s
no_features (ubuntu, stable)	56s
with_features (ubuntu, nightly, serde)	1m 14s
no_features (ubuntu, beta)	54s
with_features (macos, 1.77, serde)	1m 1s
no_features (ubuntu, nightly)	1m 16s
with_features (macos, stable, serde)	45s
no_features (macos, 1.77)	1m 14s
with_features (macos, beta, serde)	1m 3s
no_features (macos, stable)	52s
with_features (macos, nightly, serde)	1m 9s
no_features (macos, beta)	44s
with_features (windows, 1.77, serde)	1m 41s
no_features (macos, nightly)	46s
with_features (windows, stable, serde)	2m 10s
no_features (windows, 1.77)	1m 32s
with_features (windows, beta, serde)	1m 46s
no_features (windows, stable)	1m 57s
no_features (windows, beta)	1m 35s
no_features (windows, nightly)	1m 33s
	28m 27s

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