add hyperlinks to documentation and clarify derive macros (#355)

this makes it easier to navigate the documentation.

one comment seemed to suggest that the reexport of the derive
macros was depricated, however that has been ignored for several
reasons:

1. exposing macros through a derive feature flag is common practice,
   see the serde crate for an example.
2. using a feature flag means you can't accidentally have a version
   mismatch.
3. the reexport is used in several examples
4. the only evidence of deprication is a private comment,
   which would never be seen by normal users of the crate.

Co-authored-by: binarycat <[email protected]>

Author: lolbinarycat

strum/src/lib.rs

@@ -67,7 +67,7 @@ impl std::error::Error for ParseError {
 }
 
 /// This trait designates that an `Enum` can be iterated over. It can
-/// be auto generated using `strum_macros` on your behalf.
+/// be auto generated using the [`EnumIter`](derive.EnumIter.html) derive macro.
 ///
 /// # Example
 ///

strum_macros/src/lib.rs

@@ -3,6 +3,10 @@
 //! Strum is a set of macros and traits for working with
 //! enums and strings easier in Rust.
 //!
+//! This crate only contains derive macros for use with the
+//! [`strum`](https://docs.rs/strum)
+//! crate.  The macros provied by this crate are also available by
+//! enabling the `derive` feature in aforementioned `strum` crate.
 
 #![recursion_limit = "128"]
 
@@ -428,7 +432,7 @@ pub fn display(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
 /// Creates a new type that iterates of the variants of an enum.
 ///
 /// Iterate over the variants of an Enum. Any additional data on your variants will be set to `Default::default()`.
-/// The macro implements `strum::IntoEnumIterator` on your enum and creates a new type called `YourEnumIter` that is the iterator object.
+/// The macro implements [`strum::IntoEnumIterator`](https://docs.rs/strum/latest/strum/trait.IntoEnumIterator.html) on your enum and creates a new type called `YourEnumIter` that is the iterator object.
 /// You cannot derive `EnumIter` on any type with a lifetime bound (`<'a>`) because the iterator would surely
 /// create [unbounded lifetimes](https://doc.rust-lang.org/nightly/nomicon/unbounded-lifetimes.html).
 ///