docs: update awesome.spawn docs to describe implementation

Per @actionless, the awesome.spawn function now describes it's implementation on glib <2.74

spawn.c

@@ -412,15 +412,21 @@ spawn_child_exited(pid_t pid, int status)
  *
  * @tparam string|table cmd The command to launch.
  * @tparam[opt=true] boolean use_sn Use startup-notification?
- * @tparam[opt=false] boolean|string stdin Pass `true` to return a fd for stdin.
- *   Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to inherit the
- *   parent's stdin.
- * @tparam[opt=false] boolean|string stdout Pass `true` to return a fd for
- *   stdout. Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to inherit
- *   the parent's stdout.
- * @tparam[opt=false] boolean|string stderr Pass `true` to return a fd for
- *   stderr. Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to inherit
- *   the parent's stderr.
+ * @tparam[opt="DEV_NULL"] boolean|string stdin Pass `true` to return a fd for
+ *   stdin. Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to inherit
+ *   the parent's stdin. Implementation note: Pre-2.74 glib doesn't support
+ *   *explicit* `DEV_NULL`. When `DEV_NULL` is passed on glib <2.74, Awesome will
+ *   use glib's default behaviour.
+ * @tparam[opt="INHERIT"] boolean|string stdout Pass `true` to return a fd for
+ *   stdout. Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to
+ *   inherit the parent's stdout. Implementation note: Pre-2.74 glib doesn't
+ *   support *explicit* `INHERIT`. When `INHERIT` is passed on glib <2.74,
+ *   Awesome will use glib's default behaviour.
+ * @tparam[opt="INHERIT"] boolean|string stderr Pass `true` to return a fd for
+ *   stderr. Use `"DEV_NULL"` to redirect to /dev/null, or `"INHERIT"` to
+ *   inherit the parent's stderr. Implementation note: Pre-2.74 glib doesn't
+ *   support *explicit* `INHERIT`. When `INHERIT` is passed on glib <2.74,
+ *   Awesome will use glib's default behaviour.
  * @tparam[opt=nil] function exit_callback Function to call on process exit. The
  *   function arguments will be type of exit ("exit" or "signal") and the exit
  *   code / the signal number causing process termination.