Provide more and better description of the interface

Indicated that this interface _typically_ represents a URI, but
technically represents a request target per 7230.

Author: weierophinney

src/UriInterface.php

@@ -2,11 +2,31 @@
 namespace Psr\Http\Message;
 
 /**
- * Value object representing a URI.
+ * Value object representing the request target, and typically a URI.
  *
- * URIs are considered immutable; all methods that might change state MUST
- * be implemented such that they retain the internal state of the current
- * instance and return a new instance that contains the changed state.
+ * Instances of this interface  are considered immutable; all methods that
+ * might change state MUST be implemented such that they retain the internal
+ * state of the current instance and return a new instance that contains the
+ * changed state.
+ *
+ * Since this interface represents a request target per RFC 7230, the instance
+ * can potentially omit the scheme and authority. As such, test methods exist
+ * for determining what request target form is in use:
+ *
+ * - isOrigin() tests if the target is in origin-form (path + optional query
+ *   string only).
+ * - isAbsolute() tests if the target is in absolute-form (minimally scheme +
+ *   authority).
+ * - isAuthority() tests if the target contains the authority only.
+ * - isAsterisk() tests if the entirety of the target is '*'.
+ *
+ * These target forms are included, as they are valid forms for use with an
+ * HTTP request, and will appear without other URI segments available within
+ * the request line. This interface models the target as it appears.
+ *
+ * Typically, for all forms other than absolute-form, minimally the Host header
+ * will be also be present in the request message. For server-side requests,
+ * the scheme will typically be discoverable in the server parameters.
  *
  * @link http://tools.ietf.org/html/rfc3986 (the URI specification)
  * @link http://tools.ietf.org/html/rfc7230#section-2.7 (URIs as used in the HTTP specification)