docs(flashblocks): clarify fallback behavior

[skyc1e]

Adds fallback guidance for apps that use Flashblocks through public RPCs or third-party node providers.

• treats Flashblocks updates and pending reads as optimistic UX state
• lists stall, lag, and reconnect conditions where apps should fall back
• keeps receipt polling and latest reads as the source of truth for final UI state

Addresses base/base#3616

Checked:

• git diff --check
• npx mint validate (fails on an existing docs.json navigation reference to missing get-started/build-app)

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1