add relative_deadline docstring

[jakkdl]

ticks off one docstring from #3221

property added in #3010

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.17639%. Comparing base (d988edd) to head (691bf8b).
Report is 116 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##                main       #3241   +/-   ##
=============================================
  Coverage   56.17639%   56.17639%           
=============================================
  Files            246         246           
  Lines          37530       37530           
  Branches        2538        2538           
=============================================
  Hits           21083       21083           
  Misses         16319       16319           
  Partials         128         128           

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

... and 1 file with indirect coverage changes

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[A5rocks]

The whole "Read-write, :class:`float`" thing is weird but I guess we should follow conventions... Makes sense to me.

[jakkdl]

The whole "Read-write, :class:float" thing is weird but I guess we should follow conventions... Makes sense to me.

yeah I don't love it. And I don't think we're following that convention in all our docstrings. But /shrug