environ.7: Various edits to Bastien Roucariès's patch

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

man7/environ.7

@@ -280,28 +280,25 @@ and
 operations can be used to control the location of the process's environment.
 .PP
 The
-.B HOME,
-.B LOGNAME,
-.B SHELL
+.BR HOME ,
+.BR LOGNAME ,
+.BR SHELL ,
 and
 .B USER
-variables are only set when an user is changing using
-session management interface, typically by program
+variables are set when the user is changed via a
+session management interface, typically by a program such as
 .B login(1)
-from user database (for instance, but not limited, by using
-.B password (5)
-database).
-Particularly,
-.BR setuid (2)
-family of function
-does not set theses variables. Notes that as documented,
-going to root by
-.BR su (8)
+from a user database (such as
+.BR passwd (5)).
+(Switching to the root user using
+.BR su (1)
 may result in a mixed environment where
 .B LOGNAME
 and
 .B USER
-are retained from old user.
+are retained from old user; see the
+.BR su (1)
+manual page.)
 .SH BUGS
 Clearly there is a security risk here.
 Many a system command has been