Add documentation for `MaybeMissing` #98

Job	Run time
clippy	42s
linux (1.72.0)	1m 7s
cargo-deny (advisories)	21s
fmt	22s
linux (nightly)	1m 34s
cargo-deny (bans licenses sources)	19s
macos	2m 19s
windows	1m 56s
	8m 40s

Provide feedback