docs: correct appendfsync always description - main thread fsync, not background thread

[Senrian]

Summary

Fix incorrect documentation about appendfsync always in Redis docs. The original text incorrectly stated that always uses a background thread (aof_fsync) to call fsync, when in fact always runs fsync on the main thread (blocking it until the flush completes).

Changes

Root Cause

Redis 7.2 source code (aof.c try_fsync function) shows:

• AOF_FSYNC_ALWAYS → redis_fsync(server.aof_fd) called directly (main thread)
• AOF_FSYNC_EVERYSEC → aof_background_fsync(server.aof_fd) called (background thread)

Files Changed

1. docs/database/redis/redis-persistence.md: Fixed appendfsync always description to clarify main thread fsync
2. docs/database/redis/redis-common-blocking-problems-summary.md: Same fix
3. docs/database/redis/redis-questions-02.md: Same fix in config comments

Testing

• git diff --check

Closes #2697, Closes #2812