setfsuid.2: Rewrite for improved clarity and to hint history more explicitly

The current text reads somewhat clumsily. Rewrite it to introduce
the eUID and fsUID in parallel, and more clearly hint at the the
historical rationale for the fsUID, which is detailed lower in
the page.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
diff --git a/man2/setfsuid.2 b/man2/setfsuid.2
index 8918af3..ba6ad79 100644
--- a/man2/setfsuid.2
+++ b/man2/setfsuid.2
@@ -1,4 +1,5 @@
 .\" Copyright (C) 1995, Thomas K. Dyas <tdyas@eden.rutgers.edu>
+.\" and Copyright (C) 2013, 2019, Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
 .\" %%%LICENSE_START(VERBATIM)
 .\" Permission is granted to make and distribute verbatim copies of this
@@ -36,29 +37,34 @@
 .PP
 .BI "int setfsuid(uid_t " fsuid );
 .SH DESCRIPTION
-The system call
+On Linux, a process has both a filesystem user ID and an effective user ID.
+The (Linux-specific) filesystem user ID is used
+for permissions checking when accessing filesystem objects,
+while the effective user ID is used for various other kinds
+of permissions checks (see
+.BR credentials (7)).
+.PP
+Normally, the value of the process's filesystem user ID
+is the same as the value of its effective user ID.
+This is so, because whenever a process's effective user ID is changed,
+the kernel also changes the filesystem user ID to be the same as
+the new value of the effective user ID.
+A process can cause the value of its filesystem user ID to diverge
+from its effective user ID by using
 .BR setfsuid ()
-changes the value of the caller's filesystem user ID\(emthe
-user ID that the Linux kernel uses to check for all accesses
-to the filesystem.
-Normally, the value of
-the filesystem user ID
-will shadow the value of the effective user ID.
-In fact, whenever the
-effective user ID is changed,
-the filesystem user ID
-will also be changed to the new value of the effective user ID.
+to change its filesystem user ID to the value given in
+.IR fsuid .
 .PP
 Explicit calls to
 .BR setfsuid ()
 and
 .BR setfsgid (2)
-are usually used only by programs such as the Linux NFS server that
+are (were) usually used only by programs such as the Linux NFS server that
 need to change what user and group ID is used for file access without a
 corresponding change in the real and effective user and group IDs.
 A change in the normal user IDs for a program such as the NFS server
-is a security hole that can expose it to unwanted signals.
-(But see below.)
+is (was) a security hole that can expose it to unwanted signals.
+(However, this issue is historical; see below.)
 .PP
 .BR setfsuid ()
 will succeed only if the caller is the superuser or if