Export encoding function for a query parameter value

[marinelli]

After the refactoring of URL encoding (#1432), the type of the appendToQueryString function has changed.
It might be helpful to export the function encodeQueryParamValue (it was encodeQueryParam) that could be used to obtain the proper value to pass to appendToQueryString, when this function is used outside of servant.

This PR proposes to:

• move encodeQueryParam from Servant.Client.Core.HasClient to Servant.Client.Core.Request
• rename encodeQueryParam into encodeQueryParamValue, just to be more explicit
• re-export encodeQueryParamValue from Servant.Client.Core
• add short descriptions to some of the exported functions of the Servant.Client.Core.Request module