doc: shorten overlong title underlines

Title underlines should match the length of the title. Unfortunately
docutils only catches underlines that are too short.

Add some missing empty lines after titles.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>

doc/usage/cmd/askenv.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 askenv command
-===============
+==============
 
 Synopsis
 --------

doc/usage/cmd/bootdev.rst

@@ -76,7 +76,7 @@ name is provided, all hunters are run.
 
 
 bootdev select
-~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~
 
 Use this to select a particular bootdev. You can select it by the sequence
 number or name, as shown in `bootdev list`.
@@ -89,7 +89,7 @@ unselected.
 
 
 bootdev info
-~~~~~~~~~~~~~~~
+~~~~~~~~~~~~
 
 This shows information on the current bootdev, with the format looking like
 this:

doc/usage/cmd/cat.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 cat command
-===============
+===========
 
 Synopsis
 --------

doc/usage/cmd/coninfo.rst

@@ -21,7 +21,7 @@ environment variables stdin, stdout, stderr which contain a comma separated
 list of device names.
 
 Example
---------
+-------
 
 .. code-block:: console
 

doc/usage/cmd/mmc.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 mmc command
-============
+===========
 
 Synopsis
 --------

doc/usage/cmd/part.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 part command
-===============
+============
 
 Synopis
 -------

doc/usage/cmd/wdt.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 wdt command
-============
+===========
 
 Synopsis
 --------

doc/usage/cmd/xxd.rst

@@ -1,7 +1,7 @@
 .. SPDX-License-Identifier: GPL-2.0+:
 
 xxd command
-===============
+===========
 
 Synopsis
 --------