user-keyring.7: Various reworking and improvements

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

man7/user-keyring.7

@@ -18,29 +18,29 @@ Each UID the kernel deals with has its own user keyring.
 This keyring is associated with the record that the kernel maintains
 for the UID and, once created, is retained as long as that record persists.
 It is shared amongst all processes of that UID.
-.P
+
 The user keyring is created on demand when a thread requests it.
-Normally,
-this happens when
+Normally, this happens when
 .BR pam_keyinit (8)
 is invoked when a user logs in.
-.P
+
 The user keyring is not searched by default by
 .BR request_key (2).
 When
 .BR pam_keyinit (8)
 creates a session keyring, it adds to it a link to the user
 keyring so that the user keyring will be searched when the session keyring is.
-.P
+
 A special serial number value,
 .BR KEY_SPEC_USER_KEYRING ,
-is defined that
-can be used in lieu of the calling process's user keyring's actual serial
-number.
-.P
-From the keyctl utility, '\fB@u\fP' can be used instead of a numeric key ID in
+is defined that can be used in lieu of the actual serial number of
+the calling process's user keyring.
+
+From the
+.BR keyctl (1)
+utility, '\fB@u\fP' can be used instead of a numeric key ID in
 much the same way.
-.P
+
 User keyrings are independent of
 .BR clone (2),
 .BR fork (2),
@@ -51,9 +51,10 @@ and
 excepting that the keyring is destroyed when the UID record is destroyed when
 the last process pinning it exits.
 .P
-If it necessary to for a key associated with a user to exist beyond the UID
-record being garbage collected - for example for use by a cron script - then
-the
+If it is necessary for a key associated with a user to exist beyond the UID
+record being garbage collected\(emfor example, for use by a
+.BR cron (8)
+script\(emthen the
 .BR persistent-keyring (7)
 should be used instead.
 .P