Clarification on mcpu/tune/arch options

[NightStrike]

The gcc manual defines the options mcpu, mtune, and march as being
optimizers for various ARM targets.  It lists viable options, and they
are all ARM related.  However, I often see people using options that
aren't in the manual for targets that the manual says aren't
supported.  For instance, -mcpu=i686, or -march=native.  What is the
nature of this discrepancy between the documentation and what I see as
common usage?