Improve documentation on what `send_default_pii` init option should do

[rhcarvalho]

SDKs shall not send Personally Identifiable Information by default, requiring a configuration opt-in when initializing the SDK.

PII includes, but is not limited to the entire user object, auth headers, IP addresses, etc.

We document the general expectation in https://develop.sentry.dev/sdk/data-handling/#sensitive-data, but SDK developers need to inspect existing SDKs to compare implementations on what shall or shall not be covered by the send_default_pii option.

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I've also looked at these locations where more information or links could help discover the existing docs on PII:

• https://develop.sentry.dev/sdk/features/
• https://develop.sentry.dev/sdk/unified-api/#options