lib/igt_core: Fixup docs with symbolic exit codes
lib/igt_core: Fixup docs with symbolic exit codes

Conflict between me and Thomas pushing patches in parallel.

Cc: Thomas Wood <b9025d6a953b68f894a0f6a80fd2af6d55b0badd@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

dlespiau/checkmate-test-igt,tiagovignatti/intel-gpu-tools,rib/intel-gpu-tools,yipdw/intel-gpu-tools,tiagovignatti/intel-gpu-tools,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,tiagovignatti/intel-gpu-tools,erikarn/intel-gpu-tools,rib/intel-gpu-tools,dlespiau/checkmate-test-igt,chenxianqin/intel-gpu-tools,dlespiau/checkmate-test-igt,rib/intel-gpu-tools,chenxianqin/intel-gpu-tools,rib/intel-gpu-tools,dlespiau/checkmate-test-igt,erikarn/intel-gpu-tools,yipdw/intel-gpu-tools,erikarn/intel-gpu-tools,mv0/intel-gpu-tools,yipdw/intel-gpu-tools,mv0/intel-gpu-tools,yipdw/intel-gpu-tools,mv0/intel-gpu-tools,chenxianqin/intel-gpu-tools,erikarn/intel-gpu-tools,tiagovignatti/intel-gpu-tools