dr_annotation_calling_convention_t omits "cdecl"

Public enum `_dr_annotation_calling_convention_t` currently omits the `cdecl` calling convention, which is a common option on 32-bit linux. For annotation purposes the `stdcall` and `cdecl` calling conventions are equivalent (so the platform is supported), but the `dr_annotation_calling_convention_t` should include a corresponding alias, since developers have no way to know this.

The docs should also be updated to reflect the fact that `stdcall` only exists on Windows, and that `vararg` is an alias of `cdecl` on linux.