performance-optimization.md contradicts itself on enableMultiFrameCrossFilter default

[lyzhang0113]

Found while reviewing docs in PR #30, not fixed there since it's a factual claim I couldn't verify against the actual SDK behavior.

File: mrz-scanner/general/performance-optimization.md

The intro text says cross filtering is "enabled by default," and the "To disable the cross filter" lead-in plus the closing note both imply the default is true. But:

• The code comment says // default: false
• The settings table's Default column also says false

Someone who knows the actual SDK default needs to resolve which side is correct and fix the other.

[lyzhang0113]

Fixed in commit 1046cdb on faq-docs-quality-pass (PR #30): confirmed default changed to true starting v4.0.0, updated the code comment and settings table to match the intro text (also fixed a stray backtick breaking the code fence).