docs(lib): Improve maintainer facing docs (#37)

* docs(lib): Improve maintainer facing docs

Looking over the `lib.rs` file, I noticed I'd annotated the fields of
some structs, where the value was being maintained to preserve the span
from the input, but not for most. As such, this commit annotates
these remaining fields, to denote which of these are retained for these
purposes.

Alongside this, there are some minor changes to maintainer facing doc
comments explaining the elements represented by the structs, to better
reflect the representation of these fields, and the use of their
annotations.

src/lib.rs

@@ -135,12 +135,12 @@ pub fn test_gen(tokens: TokenStream) -> TokenStream {
 ///
 /// This includes:
 ///
-/// * Attributes to apply to every test case
-/// * The separator before the helper function (for preserving spans)
+/// * The attributes to apply to every test case
+/// * The separator before the helper function
 /// * The helper function for driving the tests
-/// * Arguments to pass to the helper function for every test
+/// * The arguments to pass to the helper function for every test
 /// * The default return type for the helper function
-/// * Formatting elements (for preserving spans)
+/// * The fat arrow before the braces surrounding the test cases
 /// * The values for producing the resulting test
 #[derive(Clone)]
 struct TestHelper {
@@ -335,12 +335,12 @@ impl ToTokens for Separator {
 /// This includes:
 ///
 /// * The name it will be assigned
-/// * Formatting elements (for preserving spans)
+/// * The separator colon
 /// * The arguments to specify its behaviour (see `CaseArgs` for more detail)
 #[derive(Clone)]
 struct TestCase {
     fn_name: Ident,
-    colon: Colon,
+    colon: Colon, // Preserved for span
     args: CaseArgs,
 }
 
@@ -376,13 +376,13 @@ impl ToTokens for TestCase {
 ///
 /// This includes:
 ///
-/// * Formatting elements (for preserving spans)
-/// * Attributes to apply to the specific test case
+/// * The surrounding braces
+/// * The attributes to apply to the specific test case
 /// * The arguments to pass to the helper function for the specific test case
 /// * The expected return type for the specific test case
 #[derive(Clone)]
 struct CaseArgs {
-    braces: Brace,
+    braces: Brace, // Preserved for span
     attrs: Vec<Attribute>,
     args: FnArgs,
     return_type: Option<ReturnType>,
@@ -433,7 +433,7 @@ impl ToTokens for CaseArgs {
 /// A type representing a list of arguments, and the parentheise around them.
 #[derive(Clone)]
 struct FnArgs {
-    parens: Paren,
+    parens: Paren, // Preserved for span
     args: Punctuated<Expr, Comma>,
 }