Minor docs change

[Quinn]

We should make sure to emphasize that the macro is for string literals only, and that it parses at compile time.

So this part should be altered slightly.

uuid/src/lib.rs

Lines 60 to 66 in b98c958

 //! If you have a UUID value you can use it inline: 
 //! 
 //! ``` 
 //! use uuid::{uuid, Uuid}; 
 //! 
 //! const ID: Uuid = uuid!("67e55044-10b1-426f-9247-bb680e5fe0c8"); 
 //! ``` 

... to instead say:

 //! If you have a UUID value, you can use its string literal form inline:

[Hunar Roop Kahlon]

Another way can we to have a note at the bottom warning that only string literals are accepted.

//! If you have a UUID value you can use it inline: 
 //! 
 //! ``` 
 //! use uuid::{uuid, Uuid}; 
 //! 
 //! const ID: Uuid = uuid!("67e55044-10b1-426f-9247-bb680e5fe0c8"); 
 //!
 //! __NOTE__: Only `string literal`s are accepted by the macro. If you intend on using `number literal`s, instead using `Uuid::from_u128(...);`
 //! ```