Fix TextInput placeholder documentation

[huytdps13400]

Summary

• correct the documented TextInput placeholder prop type from boolean to string
• align the generated documentation with the existing public Flow type

Fixes #2693.

Verification

• npm run build -w react-native-web-docs: passed; generated TextInput page renders placeholder as ?string
• npm run test: Flow, Prettier, ESLint, 46 DOM suites / 712 tests passed, 3 Node suites / 6 tests passed
• git diff --check: passed

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