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/autostart

@@ -12,7 +12,7 @@
 swaybg -c '#113344' >/dev/null 2>&1 &
 
 # Configure output directives such as mode, position, scale and transform.
-# Use wlr-randr to get your output names
+# Use wlr-randr to get your output names.
 # Example ~/.config/kanshi/config below:
 #   profile {
 #     output HDMI-A-1 position 1366,0