[NVPTX] Update address space mapping documentation

[rivanvx]

Summary

Current address space mapping documentation does not match the
PTXLdStInstCode::AddressSpace enum defined in lib/Target/NVPTX/NVPTX.h.
This patch updates documentation accordingly.

[jholewinski]

The documentation is actually correct here. PTXLdStInstCode is an internal enumeration that is independent of the public address space mapping. It's unfortunate that there is disagreement between the two mappings, but this should be fixed by changing the PTXLdStInstCode enum. See the AddressSpace enum in lib/Target/NVPTX/MCTargetDesc/NVPTXBaseInfo.h.

[rivanvx]

I see. Thanks for the info.

docs/NVPTXUsage.rst

 
 The NVPTX back-end uses the following address space mapping:
 
    ============= ======================
    Address Space Memory Space
    ============= ======================
    0             Generic
    1             Global
-   2             Internal Use
+   2             Constant
    3             Shared
-   4             Constant
+   4             Parameter
    5             Local
    ============= ======================
 
 Every global variable and pointer type is assigned to one of these address
 spaces, with 0 being the default address space. Intrinsics are provided which
 can be used to convert pointers between the generic and non-generic address
 spaces.