gallium/docs: improve docs for SAMPLE_POS, SAMPLE_INFO, TXQS, MSAA semantics

gallium/docs: improve docs for SAMPLE_POS, SAMPLE_INFO, TXQS, MSAA semantics

For the SAMPLE_POS and SAMPLE_INFO opcodes, clarify resource vs. render
target queries, range of postion values, swizzling, etc.  We basically
follow the DX10.1 conventions.

For the TXQS opcode and TGSI_SEMANTIC_SAMPLEID, clarify return value
and type.

For the TGSI_SEMANTIC_SAMPLEPOS system value, clarify the range of
positions returned.

v2: use 'undef' for unused vector components.  Use (0.5, 0.5, undef, undef)
for sample pos when MSAA not applicable.

v3: Add note that OPCODE_SAMPLE_INFO, OPCODE_SAMPLE_POS are not used yet
and the information is subject to change.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>