docs: clarify useWatch exact prop default behavior

[dfedoryshchev]

The exact prop row in the useWatch props table only describes what true does and leaves the default behavior undefined. The reporter in #1035 notes they repeatedly have to read source to figure out what partial matching actually matches.

Source of truth (src/logic/shouldSubscribeByName.ts:3-18):

(exact
  ? currentName === signalName
  : currentName.startsWith(signalName) || signalName.startsWith(currentName))

So when exact is false, the subscription fires whenever the subscribed name is a prefix of the changed field name or vice versa. The dedicated test at src/__tests__/logic/shouldSubscribeByName.test.ts:9-12 pins this behavior (including the raw-string-prefix edge case where shouldSubscribeByName('test1', 'test') is truthy). Call site at createFormControl.ts:1196 invokes the function directly with no path normalization in between, so the prefix semantics apply end-to-end.

Single-row content change in src/content/docs/usewatch.mdx: replace the current one-liner with a description that states the default behavior in terms of the actual prefix check, with one concrete example for the typical nested-path case. No prettier reflow (the existing compute row already defines the column width).

Closes #1035.

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