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.

src/window-rules.c

@@ -69,8 +69,8 @@ window_rules_get_property(struct view *view, const char *property)
 	 * for foot's "serverDecoration" property to be "default".
 	 *
 	 *     <windowRules>
-	 *       <windowRule identifier="*" serverDecoration="no"/>
-	 *       <windowRule identifier="foot" serverDecoration="default"/>
+	 *       <windowRule identifier="*" serverDecoration="no" />
+	 *       <windowRule identifier="foot" serverDecoration="default" />
 	 *     </windowRules>
 	 */
 	struct window_rule *rule;