efibootmgr: Document -e command line switch

-e env will include `env` in the boot loader. Document that the boot
loader appends the `env` to the BootXXXX variable, and will parse it as
a series of a=b values to set in the boot loader's environment.  These
assignments are separated by spaces. The env arg needs to be quoted if
more than one env var is to be set (we parse only the next argument on
the command line).

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usr.sbin/efibootmgr/efibootmgr.8

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 26, 2023
+.Dd December 17, 2023
 .Dt EFIBOOTMGR 8
 .Os
 .Sh NAME
@@ -130,6 +130,21 @@ Create a new
 variable (aka method or entry).
 .It Fl D -dry-run
 Process but do not change any variables.
+.It Fl e -env
+When creating a
+.Cm Boot
+variable, include a set of environment variables for the loader to set after
+parsing the command line.
+Variables are of the form
+.Dq a=b
+and separated by spaces.
+The argument should be quoted.
+.Nm appends these to the end of the
+.Cm Boot
+environment variable.
+Before processing anything else,
+.Xr loader.efi 8
+will parse this area and set all variables found there.
 .It Fl E -esp
 Print the
 .Fx
@@ -216,16 +231,16 @@ the EFI partition mounted under
 .Pa /boot/efi ,
 mark the method active, using
 the given loader and label the method
-.Qq FreeBSD-11 :
+.Qq FreeBSD-15 :
 .Pp
-.Dl efibootmgr -a -c -l /boot/efi/EFI/freebsd/loader.efi -L FreeBSD-11
+.Dl efibootmgr -a -c -l /boot/efi/EFI/freebsd/loader.efi -L FreeBSD-15
 .Pp
 This will result in the next available bootnum being assigned to a
 new UEFI boot variable, and given the label
-.Qq FreeBSD-11
+.Qq FreeBSD-15
 such as:
 .Pp
-.Dl Boot0009 FreeBSD-11
+.Dl Boot0009 FreeBSD-15
 .Pp
 Note newly created boot entries are, by default, created inactive, hence
 the reason