Updated docstring for from_thread_run to reflect how to get trio token

[oakal510]

Added a documentation fix for the following issue: fixes https://github.com/python-trio/trio/issues/1439.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.63%. Comparing base (6f62575) to head (23b2c87).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3001 +/- ## ======================================= Coverage 99.63% 99.63% ======================================= Files 120 120 Lines 17798 17798 Branches 3198 3198 ======================================= Hits 17733 17733 Misses 46 46 Partials 19 19 ``` | [Files](https://app.codecov.io/gh/python-trio/trio/pull/3001?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_threads.py](https://app.codecov.io/gh/python-trio/trio/pull/3001?src=pr&el=tree&filepath=src%2Ftrio%2F_threads.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3RocmVhZHMucHk=) | `100.00% <ø> (ø)` | |

[trio-bot[bot]]

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