Document that start is run as soon as SchedulerPlugin is registered

[oxinabox]

I was confused as to how SchedulerPlugin's start method would be run.
Since module some advanced stuff like preload scripts, most of the time by the time you have access to a Client object to call register_plugin on, the Scheduler has already started.
Then i checked the code and saw that start is also run when a plugin is registered.

distributed/distributed/scheduler.py

Line 6261 in dc182bd

result = plugin.start(self)

• Tests added / passed
  (N/A docs only)
• Passes pixi run lint

[github-actions]

Unit Test Results

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    40 files  ±0      40 suites  ±0   14h 8m 46s ⏱️ - 21m 9s
 4 160 tests ±0   3 981 ✅ +2    178 💤 ±0  1 ❌  - 2 
80 960 runs  ±0  76 720 ✅ +2  4 239 💤 ±0  1 ❌  - 2 

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Results for commit 6a45d6e. ± Comparison against base commit dc182bd.