docs(bfcache): add note about Back/Forward Cache (BFCache) limitations and default behavior

[gitKashish]

What does this PR do?

This PR adds a brief warning to the Pages guide (docs/pages.md) and the API reference for page.goBack() / page.goForward() regarding the Back/Forward Cache (BFCache). It clarifies two things:

1. Playwright disables BFCache by default.
2. Even if explicitly enabled, testing BFCache restorations is not supported due to network-level lifecycle limitations.

Why is this needed?

I recently spent quite a bit of time going down a rabbit hole trying to write an E2E test for a BFCache bug I was fixing in an open-source project.

First, I struggled to figure out why the cache wasn't triggering at all (before realizing it was disabled by default in Playwright). Then, once I managed to trigger it, I couldn't figure out why page.goBack() was consistently timing out or leaving the page in a desynchronized state.

I eventually realized that because BFCache restores unfreeze the DOM without firing standard network-level navigation events (like commit or load), Playwright's internal Page lifecycle tracking gets completely bypassed. (I documented my deep-dive into this limitation in this comment on my Forgejo PR, which ultimately led to me dropping the E2E test entirely).

I thought adding a small note to the documentation might save other developers the hours of debugging I went through when trying to test this specific browser feature!

Feedback requested

I want to defer to the maintainers on this, if you feel there is a better way to phrase this, a better place to put it (e.g., only in the API docs vs the guides), or if you feel this is too much of an edge case to include in the core documentation at all, please let me know.

I am completely open to making adjustments or closing the PR if it doesn't align with the documentation goals. Thanks for taking a look!

[gitKashish]

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[dcrousso]

thanks for the fix!

[dcrousso]

i think you need to add this to packages/playwright-client/types/types.d.ts (and potentially also packages/playwright-core/types/types.d.ts too)