Log Message:
tex-unicode-math{,-doc}: update to 0.8m

* Restore behaviour of legacy syntax `x_\mathrm{x}` (i.e., with no braces).
  While strictly ‘incorrect’, this usage is widely used.
  N.B. this syntax is not supported for the `\symXX` commands.

* Add `\cuberoot` and `\fourthroot` as proper radicals.

* Some additional input subscripts/superscripts.

* Some documentation additions (thanks for the contributions).

* Deprecation warning: the *internal* `\__um_switchto_XX:` commands
  will be dropped in a future release in favour of `\__um_switch_to:n {XX}`
  (or similar). These are internal commands and shouldn't be used by
  third-parties; please write if you have a use case.