Document what feature everything requires
diff --git a/src/lib.rs b/src/lib.rs
index 948dd3e..7d236db 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -418,6 +418,8 @@
     ///     # fn visit_bin_op(&mut self, node: &'ast BinOp);
     /// }
     /// ```
+    ///
+    /// *This module is available if Syn is built with the `"visit"` feature.*
     #[cfg(feature = "visit")]
     pub mod visit;
 
@@ -453,6 +455,9 @@
     ///     # fn visit_bin_op_mut(&mut self, node: &mut BinOp);
     /// }
     /// ```
+    ///
+    /// *This module is available if Syn is built with the `"visit-mut"`
+    /// feature.*
     #[cfg(feature = "visit-mut")]
     pub mod visit_mut;
 
@@ -489,6 +494,8 @@
     ///     # fn fold_bin_op(&mut self, node: BinOp) -> BinOp;
     /// }
     /// ```
+    ///
+    /// *This module is available if Syn is built with the `"fold"` feature.*
     #[cfg(feature = "fold")]
     pub mod fold;
 
@@ -528,6 +535,8 @@
 ///
 /// [`syn::parse2`]: fn.parse2.html
 ///
+/// *This function is available if Syn is built with the `"parsing"` feature.*
+///
 /// # Examples
 ///
 /// ```rust
@@ -577,6 +586,8 @@
 ///
 /// [`Macro`]: struct.Macro.html
 /// [`syn::parse`]: fn.parse.html
+///
+/// *This function is available if Syn is built with the `"parsing"` feature.*
 #[cfg(feature = "parsing")]
 pub fn parse2<T>(tokens: proc_macro2::TokenStream) -> Result<T, ParseError>
 where
@@ -605,6 +616,8 @@
 
 /// Parse a string of Rust code into the chosen syntax tree node.
 ///
+/// *This function is available if Syn is built with the `"parsing"` feature.*
+///
 /// # Examples
 ///
 /// ```rust
@@ -643,6 +656,8 @@
 ///
 /// If present, either of these would be an error using `from_str`.
 ///
+/// *This function is available if Syn is built with the `"parsing"` feature.*
+///
 /// # Examples
 ///
 /// ```rust,no_run
@@ -725,6 +740,9 @@
 /// }
 /// ```
 ///
+/// *This macro is available if Syn is built with both the `"parsing"` and
+/// `"printing"` features.*
+///
 /// # Example
 ///
 /// The following helper function adds a bound `T: HeapSize` to every type