Add documentation on DocTest and RunnableDoctest structs

src/librustdoc/doctest.rs

@@ -433,6 +433,7 @@ fn wrapped_rustc_command(rustc_wrappers: &[PathBuf], rustc_binary: &Path) -> Com
     command
 }
 
+/// This struct contains information needed for running a doctest.
 struct RunnableDoctest {
     full_test_code: String,
     full_test_line_offset: usize,

src/librustdoc/doctest/make.rs

@@ -17,6 +17,8 @@ use rustc_span::FileName;
 
 use super::GlobalTestOptions;
 
+/// This struct contains information about the doctest itself which is then used to generate
+/// doctest source code appropriately.
 pub(crate) struct DocTest {
     pub(crate) supports_color: bool,
     pub(crate) already_has_extern_crate: bool,