SVM: Add doc comments, restrict visibility of some xfaces to crate (#… #15

Job	Run time
android (ubuntu-20.04, x86_64-linux-android)	2m 51s
ios (macos-11, aarch64-apple-ios)	6m 24s
android (ubuntu-20.04, aarch64-linux-android)	3m 0s
ios (macos-11, x86_64-apple-ios)	6m 6s
ios (macos-11, aarch64-apple-darwin)	6m 41s
android (ubuntu-20.04, i686-linux-android)	3m 1s
ios (macos-11, x86_64-apple-darwin)	7m 12s
android (ubuntu-20.04, armv7-linux-androideabi)	3m 13s
error_reporting	0s
	38m 28s

Provide feedback