getrandom.2: Rework discussion of blocking section on interuption by signals

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/getrandom.2

@@ -155,7 +155,7 @@ would have blocked if the
 flag was not set.
 .TP
 .B EINTR
-While blocked waiting for entropy, the call was interrupted by a signal
+The call was interrupted by a signal
 handler; see the description of how interrupted
 .BR read (2)
 calls on "slow" devices are handled with and without the
@@ -190,22 +190,29 @@ When a sufficient number of random bits has been collected, the
 entropy pool is considered to be initialized.
 This state is normally reached early in the system bootstrap phase.
 .SS Interruption by a signal handler
-A call to
-.BR getrandom ()
-can block only when called without the
-.B GRND_NONBLOCK
-flag.
 When reading from
 .I /dev/urandom
 .RB ( GRND_RANDOM
 is not set),
-blocking can occur if the entropy pool has not been initialized yet
-or if the requested number of bytes is large.
+.BR getrandom ()
+will block until the entropy pool has been initialized
+(unless the
+.BR GRND_NONBLOCK
+flag was specified).
+If a request is made to read a large number (more than 256) of bytes,
+.BR getrandom ()
+will block until those bytes have been generated and transferred
+from kernel memory to
+.IR buf .
 When reading from
 .I /dev/random
 .RB ( GRND_RANDOM
 is set),
-blocking occurs if not enough random bytes are available.
+.BR getrandom ()
+will block until some random bytes become available
+(unless the
+.BR GRND_NONBLOCK
+flag was specified).
 
 The behavior when a call to
 .BR getrandom ()