thread-keyring.7: Various rewordings and additions

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man7/thread-keyring.7

@@ -15,27 +15,32 @@ thread-keyring \- per-thread keyring
 .SH DESCRIPTION
 The thread keyring is a keyring used to anchor keys on behalf of a process.
 It is created only when a thread requests it.
-.P
+
 A special serial number value,
 .BR KEY_SPEC_THREAD_KEYRING ,
-is defined that
-can be used in lieu of the calling thread's thread keyring's actual serial
-number.
-.P
-From the keyctl utility, '\fB@t\fP' can be used instead of a numeric key ID in
-much the same way, but as keyctl is a program run after forking, this is of no
-utility.
-.P
+is defined that can be used in lieu of the actual serial number of
+the calling thread's thread keyring.
+
+From the
+.BR keyctl (1)
+utility, '\fB@t\fP' can be used instead of a numeric key ID in
+much the same way, but as
+.BR keyctl (1)
+is a program run after forking, this is of no utility.
+
 Thread keyrings are not inherited across
 .BR clone (2)
+and
+.BR fork (2)
 and are cleared by
 .BR execve (2).
-A thread keyring is destroyed when the thread that refers to it exits.
-.P
-If a thread doesn't have a thread keyring when it is accessed, then the thread
-keyring will be created if the keyring is to be modified, otherwise error
-.B ENOKEY
-will be issued.
+A thread keyring is destroyed when the thread that refers to it terminates.
+
+Initially, a thread does not have a thread keyring.
+If a thread doesn't have a thread keyring when it is accessed,
+then it will be created if it is to be modified;
+otherwise the operation will fail with the error
+.BR ENOKEY .
 .SH SEE ALSO
 .ad l
 .nh