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/theme.c

@@ -788,7 +788,7 @@ entry(struct theme *theme, const char *key, const char *value)
 			value, "window.button.spacing");
 	}
 
-	/* botton hover overlay */
+	/* button hover overlay */
 	if (match_glob(key, "window.button.hover.bg.color")) {
 		parse_color(value, theme->window_button_hover_bg_color);
 	}