docs: explain syncProcessCwd async contexts

[KirtiRamchandani]

Problem

Issue #876 notes that the current docs mention cd() and syncProcessCwd(), but do not explain the async-context mismatch that can happen when cd() changes the global process directory while within() restores only zx's internal $ context.

Root cause

The syncProcessCwd() section only states what the hook does. It does not show how process.cwd() can drift from $ in separate callbacks, or when users should prefer $.cwd instead.

Solution

Add examples showing the mismatch without the hook and the restored behavior with syncProcessCwd() enabled, plus guidance to use $.cwd for zx-only directory changes.

Tests run

• npm exec --yes prettier -- --check docs/api.md
• git diff --check

Fixes #876

[antonmedv]

@antongolub looks right and explanatory to me. ptal