Add documentation_style to with short and full options

tests/rust/doclength_short.rs

@@ -0,0 +1,16 @@
+/// The root of all evil.
+///
+/// But at least it contains some more documentation as someone would expect
+/// from a simple test case like this. Though, this shouldn't appear in the
+/// output.
+#[no_mangle]
+pub extern "C" fn root() {
+}
+
+/// A little above the root, and a lot more visible, with a run-on sentence
+/// to test going over the first line.
+///
+/// Still not here, though.
+#[no_mangle]
+pub extern "C" fn trunk() {
+}

tests/rust/doclength_short.toml

@@ -0,0 +1 @@
+documentation_length = "short"