Documentation on how to use context on client. Why contextKey type?

After adding the middleware `router.Use(oauth.Authorize("mySecretKey-10101", nil))` and being able to authorize a user, I had some issues figuring out how to access the information of the access token using the Context.

The middleware.go file adds the token information to the Context using a new type that is basically a string. Code in question:
`type contextKey string

const (
CredentialContext contextKey = "oauth.credential"
ClaimsContext contextKey = "oauth.claims"
ScopeContext contextKey = "oauth.scope"
TokenTypeContext contextKey = "oauth.tokentype"
AccessTokenContext contextKey = "oauth.accesstoken"
)`

Given that the context is accessed using `r.Context().Value(key)` and that key is basically a string, I thought I had to access the claims just by using `r.Context().Value("oauth.claims")` but this approach returned nil.

To successfully access the token information in the context I had to import the oauth package and use the consts that I put above since their type is not a string but contextKey. Working Example:
`r.Context().Value(oauth.ClaimsContext)`

I was wondering why this contextKey type is used instead of using a string

Start with middleware.go and the contextKey, CredentialContext, ClaimsContext, ScopeContext, TokenTypeContext, and AccessTokenContext definitions mentioned in the issue. Document why the typed keys are used and show that callers should use the exported constants, with an example matching the reported context access; the documentation should answer why a plain string returns nil.

Written by the indexing model from the issue text.