update-initramfs: Consistently show which options take arguments

The text in the manual page implies that -b needs an argument
but the usage summaries don't say that.

While we're at it, remove the brackets in '-k [version]', as
these suggest the version argument is optional.

Closes: #762768
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

update-initramfs

@@ -45,12 +45,12 @@ usage()
 Usage: ${0} [OPTION]...
 
 Options:
- -k [version]	Specify kernel version or 'all'
+ -k version	Specify kernel version or 'all'
  -c		Create a new initramfs
  -u		Update an existing initramfs
  -d		Remove an existing initramfs
  -t		Take over a custom initramfs with this one
- -b		Set alternate boot directory
+ -b directory	Set alternate boot directory
  -v		Be verbose
  -h		This message
 

update-initramfs.8

@@ -10,7 +10,8 @@ update\-initramfs \- generate an initramfs image
 .IR version ]
 .RB [ \-t ]
 .RB [ \-v ]
-.RB [ \-b ]
+.RB [ \-b
+.IR directory ]
 .RB [ \-h ]
 .SH DESCRIPTION
 The
@@ -62,7 +63,7 @@ This option increases the amount of information you are given during
 the chosen action.
 
 .TP
-\fB \-b
+\fB \-b \fI directory
 Set an different bootdir for the image creation.
 
 .TP