docs(custom-handler): clarify IsInstalled call sites and authoring guidance (#750)

[Nox-MSFT]

Summary

Resolves #750. The IsInstalled row in docs/agent-reference/how-to-implement-custom-update-handler.md previously said only "In some situation, an Agent workflow can invoke 'IsInstalled'…" without explaining which situations, which is exactly what the issue reported.

Changes

docs/agent-reference/how-to-implement-custom-update-handler.md:

1. Rewrote the IsInstalled table row to state the actual contract: it is a probe that may be called multiple times per deployment and must be cheap, side-effect-free, and idempotent; it compares the workflow's installedCriteria against handler-scoped state and must not consult system-wide state. Also fixed a malformed <br/ and clarified that a ResultCode of 0 is ADUC_Result_Failure (which the Steps Handler treats as NotInstalled).

2. Added an "IsInstalled call sites and authoring guidance" section that:

   • enumerates the known call sites (agent core before a deployment and after a successful Apply; Steps Handler before each step's Download and before Install/Apply; and the aggregate call after the per-(component × step) loop);
   • lists authoring rules (be cheap / scoped / deterministic, use installedCriteria, don't return 0 to mean "installed", no reboots/restarts);
   • documents the conjunction semantics when used as a step in a multi-step manifest and cross-links steps-handler.md → IsInstalled.

Documentation-only change (+24 / −1); no code is affected.

Fixes #750