Add vectorcall and fastcall explanation
@@ -1678,8 +1678,10 @@ There are also some platform-specific ABI strings:
* `extern "stdcall"` -- The default for the Win32 API on x86\_32.
* `extern "win64"` -- The default for C code on x86\_64 Windows.
* `extern "aapcs"` -- The default for ARM.
-* `extern "fastcall"`
-* `extern "vectorcall"`
+* `extern "fastcall"` -- The `fastcall` ABI -- corresponds to MSVC's
+ `__fastcall` and GCC and clang's `__attribute__((fastcall))`
+* `extern "vectorcall"` -- The `vectorcall` ABI -- corresponds to MSVC's
+ `__vectorcall` and clang's `__attribute__((vectorcall))`
Finally, there are some rustc-specific ABI strings:
