Index: usr.bin/sed/sed.1
===================================================================
--- usr.bin/sed/sed.1
+++ usr.bin/sed/sed.1
@@ -31,7 +31,7 @@
 .\"	@(#)sed.1	8.2 (Berkeley) 12/30/93
 .\" $FreeBSD$
 .\"
-.Dd June 10, 2020
+.Dd March 8, 2021
 .Dt SED 1
 .Os
 .Sh NAME
@@ -41,15 +41,15 @@
 .Nm
 .Op Fl Ealnru
 .Ar command
-.Op Fl I Ar extension
-.Op Fl i Ar extension
+.Op Fl I Ns Ar extension
+.Op Fl i Ns Ar extension
 .Op Ar
 .Nm
 .Op Fl Ealnru
 .Op Fl e Ar command
 .Op Fl f Ar command_file
-.Op Fl I Ar extension
-.Op Fl i Ar extension
+.Op Fl I Ns Ar extension
+.Op Fl i Ns Ar extension
 .Op Ar
 .Sh DESCRIPTION
 The
@@ -102,16 +102,33 @@
 .Ar command_file
 is
 .Dq Li - .
-.It Fl I Ar extension
+.It Fl I Ns Ar extension
 Edit files in-place, saving backups with the specified
 .Ar extension .
-If a zero-length
-.Ar extension
-is given, no backup will be saved.
-It is not recommended to give a zero-length
+.Pp
+There should be no space between
+.Fl I
+and the
+.Ar extension ,
+for compatibility with versions of
+.Nm
+available on other operating systems, and with future versions of
+.Fx .
+.Pp
+Backups are optional, but recommended as otherwise you risk corruption or
+partial content in situations where disk space is exhausted, etc.
+To avoid creating a backup, specify a zero-length
 .Ar extension
-when in-place editing files, as you risk corruption or partial content
-in situations where disk space is exhausted, etc.
+by using
+.Fl I
+"".
+This is a special exception to the note about avoiding space between
+.Fl I
+and the
+.Ar extension .
+This usage is also incompatible with the
+.Nm
+available on other operating systems.
 .Pp
 Note that in-place editing with
 .Fl I
@@ -128,7 +145,7 @@
 where using
 .Fl i
 is desired.
-.It Fl i Ar extension
+.It Fl i Ns Ar extension
 Edit files in-place similarly to
 .Fl I ,
 but treat each file independently from other files.