fix(develop): Clarify add_feature_flag API value type requirements

[Lms24]

DESCRIBE YOUR PR

Update the add_feature_flag API documentation to specify that only boolean values are accepted.

Looks like the initial spec was too broad, as our product currently only supports boolean feature flags.
This was raised in getsentry/sentry-javascript#21157.

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Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview, Comment	Jun 5, 2026 4:21pm

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Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ignored	Preview	Jun 5, 2026 4:21pm