docs: Document not to use `volatile` qualifiers

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #600
11 jobs for 600-drop-volatile in 13 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Style Check
passed #997306
check-todos

00:01:55

failed #997304
allowed to fail
sh-check

00:01:08

failed #997305
allowed to fail
style-check-diff

00:02:36

 
  Build
passed #997309
cross-android_api21_arm64

00:01:48

passed #997310
cross-android_api28_arm64

00:01:36

passed #997311
cross-mingw64

00:02:34

passed #997308
debian-stable-x86_64

00:02:21

passed #997307
fedora-x86_64

00:08:44

passed #997312
win32-ps
msys2-mingw32

00:12:06

passed #997313
win32-ps
vs2017-x64

00:06:28

 
  Coverage
passed #997314
coverage

00:01:39

76.2%
 
Name Stage Failure
failed
style-check-diff Style Check
+          g_print (".%c", 'a' + test->id);
+ g_thread_yield (); /* force context switch */
+ }
+ }

return NULL;
}
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
sh-check Style Check
                                ^-- SC1117: Backslash is literal in "\e". Prefer explicit escaping: "\\e".


In glib/update-pcre/update.sh line 55:
cd ..
^-- SC2103: Use a ( subshell ) to avoid having to cd back.

Cleaning up file based variables
ERROR: Job failed: exit code 1