docs(watchers): clarify watchEffect dependency re-collection

[smg99]

Description of Problem

watchEffect() dependency tracking wording can be misread as a fixed dependency list. The docs should clarify that dependencies are collected on each synchronous run, so branch-dependent access can change the tracked set between runs.

Proposed Solution

Update the watchEffect() tip in src/guide/essentials/watchers.md to clarify:

• tracking happens during each synchronous run;
• dependencies are re-collected on every re-run based on properties accessed in that run;
• conditional branches can change the tracked dependency set between runs;
• async effects still only track accesses before the first await tick.

Additional Information

Context: vuejs/core#13688 and vuejs/core#15212

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