SVM: Add doc comments, restrict visibility of some xfaces to crate #357

Job	Run time
test	0s
cargo-test-sbf (token/program)	7m 58s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	12m 53s
check	0s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	7m 52s
cargo-test-sbf (token-upgrade/program)	9m 56s
cargo-test-sbf (feature-proposal/program)	7m 59s
cargo-test-sbf (governance/addin-mock/program, governance/program)	12m 36s
cargo-test-sbf (memo/program)	7m 43s
cargo-test-sbf (name-service/program)	8m 17s
cargo-test-sbf (stake-pool/program)	10m 27s
cargo-test-sbf (single-pool/program)	10m 31s
	1h 36m 12s

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