Spell out fail fast semantics

doc/fail_fast.md

+gRPC Fail Fast Semantics
+========================
+
+Fail fast requests allow terminating requests (with status UNAVAILABLE) prior
+to the deadline of the request being met.
+
+gRPC implementations of fail fast can terminate requests whenever a channel is
+in the TRANSIENT_FAILURE or SHUTDOWN states. If the channel is in any other
+state (CONNECTING, READY, or IDLE) the request should not be terminated.
+
+Fail fast SHOULD be the default for gRPC implementations, with an option to
+switch to non fail fast.
+
+The opposite of fail fast is 'ignore connectivity'.
+