SVM: Add doc comments, restrict visibility of some xfaces to crate (#… #161

Job	Run time
ios (macos-11, aarch64-apple-ios)	6m 13s
android (ubuntu-20.04, x86_64-linux-android)	2m 53s
ios (macos-11, x86_64-apple-ios)	7m 18s
android (ubuntu-20.04, aarch64-linux-android)	3m 8s
ios (macos-11, aarch64-apple-darwin)	5m 57s
android (ubuntu-20.04, i686-linux-android)	2m 58s
ios (macos-11, x86_64-apple-darwin)	6m 23s
android (ubuntu-20.04, armv7-linux-androideabi)	3m 22s
error_reporting	0s
	38m 12s

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