Document that trio.sleep_forever will raise RuntimeError instead of returning

[A5rocks]

Fixes #3113.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.59632%. Comparing base (8850705) to head (1c94d82).
Report is 304 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main       #3115         +/-   ##
===================================================
+ Coverage   99.58694%   99.59632%   +0.00938%     
===================================================
  Files            121         121                 
  Lines          18157       18827        +670     
  Branches        3272        3517        +245     
===================================================
+ Hits           18082       18751        +669     
- Misses            52          53          +1     
  Partials          23          23                 

Files with missing lines	Coverage Δ
src/trio/_timeouts.py	100.00000% <ø> (ø)

... and 10 files with indirect coverage changes

[TeamSpen210]

I don't think this should say "instead of returning", it won't do so in normal usage, it'll only return if cancelled. Better to say that it never returns, and raises RuntimeError if manually rescheduled.