doc/flux_future_get(3): simplify descriptions

Problem: flux_future_get(3) includes some details that
are not likely to be needed by most users, and this
makes it hard to understand.

Drop some obscure details that are more or less
covered in flux_future_create(3).

doc/man3/flux_future_get.adoc

@@ -49,34 +49,23 @@ DESCRIPTION
 future is not yet fulfilled, it calls `flux_future_wait_for()` internally
 with a negative _timeout_, causing it to block until the future is fulfilled.
 A pointer to the result is assigned to _result_ (caller must NOT free),
-or -1 is returned if the future was fulfilled with an error.  Often this
-function is wrapped with a class-specific result access function.
+or -1 is returned if the future was fulfilled with an error.
 
 `flux_future_then()` sets up a continuation callback _cb_ that is called
 with opaque argument _arg_ once the future is fulfilled.  The continuation
-is registered on the reactor passed to a class-specific create function
-(some create functions accept a flux_t handle, and the reactor is
-derived from that).
-
-The continuation will normally use `flux_future_get()` or a class-specific
-access function to obtain the result from the future container without
-blocking.  The continuation may call `flux_future_destroy()` or
-`flux_future_reset()`.  It is not an error to set up a continuation on
-a future that has already been fulfilled.  If _timeout_ is non-negative,
-the future must be fulfilled within the specified amount of time or the
-timeout fulfills it with an error (errno set to ETIMEDOUT).  The reactor
-must be run or re-entered in order for the timer and the future activity
-to make progress.
+will normally use `flux_future_get()` or a class-specific access function
+to obtain the result from the future container without blocking.  The
+continuation may call `flux_future_destroy()` or `flux_future_reset()`.
+If _timeout_ is non-negative, the future must be fulfilled within the
+specified amount of time or the timeout fulfills it with an error (errno
+set to ETIMEDOUT).
 
 `flux_future_wait_for()` blocks until the future is fulfilled, or _timeout_
 (if non-negative) expires.  This function may be called multiple times,
 with different values for _timeout_.  If the timeout expires before
 the future is fulfilled, an error is returned (errno set to ETIMEDOUT)
-but the future remains unfulfilled.  The timer and the future activity can
-make progress while `flux_future_wait_for()` is executing, unless _timeout_
-is zero, in which case the function times out immediately if the future
-has not already been fulfilled.  While `flux_future_wait_for()` is executing,
-unrelated reactor watchers and message handlers are not active.
+but the future remains unfulfilled.  If _timeout_ is zero, function times
+out immediately if the future has not already been fulfilled.
 
 `flux_future_reset()` unfulfills a future, invalidating any result stored
 in the container, and preparing it to be fulfilled once again.  If a