Typo/style fixes in docs and code comments

Codespell(1)-pointed typos in files listed by `git ls-files`
(sans checkpatch.pl and possibly some others). Removed some
extra spaces. Added a few missing trailing periods.
`Default is ...`.  Added spaces in ' />' where missing
(sans e.g. wayland protocol and t/* files).
Fit some lines in docs/*.scd to 80 colums.

Used git grep commands (to find similar cases):
$ git grep -n '\S/>'
$ git grep -nF '.  '
$ git grep -n '[^\t*'\'',{#]\t'
$ git grep -ni 'default '

No functional change. No change in *.[ch] line numbers.

docs/menu.xml

@@ -23,7 +23,7 @@
     Any menu with the id "workspaces" will be hidden
     if there is only a single workspace available.
   -->
-  <menu id="client-send-to-menu"/>
+  <menu id="client-send-to-menu" />
   <!--
     openbox default workspace selector
     to use replace above workspace menu with the example below
@@ -56,9 +56,9 @@
   # A prompt can be used as follows:
   <item label="Exit">
     <action name="If">
-      <prompt message="Do you really want to exit the compositor?"/>
+      <prompt message="Do you really want to exit the compositor?" />
       <then>
-        <action name="Exit"/>
+        <action name="Exit" />
       </then>
     </action>
   </item>