man/io_uring_prep_linkat.3: expand ERRORS, NOTES and add examples

Add comprehensive documentation including:

ERRORS section with common error codes:
- -EEXIST, -ENOENT, -EPERM, -EXDEV
- -EACCES, -EMLINK, -ENOSPC, -EROFS

NOTES section expanded with:
- Flags documentation (AT_SYMLINK_FOLLOW, AT_EMPTY_PATH)
- Explanation of hard links vs symbolic links
- Hard link limitations

EXAMPLE section with practical code samples:
- Create a hard link
- Create hard link following symlinks
- Create link relative to directories
- Safe file update pattern using hard link

Also expanded SEE ALSO with related functions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>