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/menu/menu.c

@@ -172,7 +172,7 @@ item_parse_accelerator(struct menuitem *item, const char *text)
 			accel_ptr = underscore + 1;
 			break;
 		} else {
-			/* Ignore empty accelertor */
+			/* Ignore empty accelerator */
 			break;
 		}
 	}
@@ -627,7 +627,7 @@ fill_menu(struct menu *parent, xmlNode *n)
 			 *
 			 * <?xml version="1.0" encoding="UTF-8"?>
 			 * <openbox_menu>
-			 *   <menu id="root-menu" label="foo" execute="bar"/>
+			 *   <menu id="root-menu" label="foo" execute="bar" />
 			 * </openbox_menu>
 			 */
 		} else {