MFC 359465: Document EINTEGRITY errors for many system calls.

EINTEGRITY was previously documented as a UFS-specific error for
mount(2). This documents EINTEGRITY as a filesystem-independent error
that may be reported by the backing store of a filesystem.

While here, document EIO as a filesystem-independent error for both
mount(2) and posix_fadvise(2). EIO was previously only documented for
UFS for mount(2).