rebase: update comment about FreeBSD /bin/sh

Commit 9f50d32b introduced a fix for FreeBSD /bin/sh misbehaviour
when dot-sourcing a file containing "return" statements outside of
any function, from a function in another shell script. That issue
affects FreeBSD 9.x, and is not present in the /bin/sh in FreeBSD
10.3 and later. Update the comment to clarify this.

The example from 9f50d32b's commit message produces the expected output
on FreeBSD 10.3 and -CURRENT (the upcoming 11.0):

% sh script1.sh
only this line should show
%
Signed-off-by: NEd Maste <emaste@freebsd.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-rebase--am.sh

@@ -9,8 +9,8 @@
 # below were not inside any function, and expected to return
 # to the function that dot-sourced us.
 #
-# However, FreeBSD /bin/sh misbehaves on such a construct and
-# continues to run the statements that follow such a "return".
+# However, older (9.x) versions of FreeBSD /bin/sh misbehave on such a
+# construct and continue to run the statements that follow such a "return".
 # As a work-around, we introduce an extra layer of a function
 # here, and immediately call it after defining it.
 git_rebase__am () {

git-rebase--interactive.sh

@@ -838,8 +838,8 @@ add_exec_commands () {
 # below were not inside any function, and expected to return
 # to the function that dot-sourced us.
 #
-# However, FreeBSD /bin/sh misbehaves on such a construct and
-# continues to run the statements that follow such a "return".
+# However, older (9.x) versions of FreeBSD /bin/sh misbehave on such a
+# construct and continue to run the statements that follow such a "return".
 # As a work-around, we introduce an extra layer of a function
 # here, and immediately call it after defining it.
 git_rebase__interactive () {

git-rebase--merge.sh

@@ -107,8 +107,8 @@ finish_rb_merge () {
 # below were not inside any function, and expected to return
 # to the function that dot-sourced us.
 #
-# However, FreeBSD /bin/sh misbehaves on such a construct and
-# continues to run the statements that follow such a "return".
+# However, older (9.x) versions of FreeBSD /bin/sh misbehave on such a
+# construct and continue to run the statements that follow such a "return".
 # As a work-around, we introduce an extra layer of a function
 # here, and immediately call it after defining it.
 git_rebase__merge () {