[clang] [clang] [doc] Added documentation to intrinsics in cpuid.h (PR #170507)

[Phoebe Wang via cfe-commits]

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@@ -278,6 +278,23 @@
         : "0"(__leaf), "2"(__count))
 #endif
 
+/// Queries the processor to determine the highest supported \c CPUID leaf.
+///
+/// \headerfile <cpuid.h>
+///
+/// This intrinsic corresponds to the <c> CPUID </c> instruction.
+///
+/// \param __leaf
+///    \a __leaf can be either 0x0 or 0x8000000. If \a __leaf == 0x0, the
+///    highest supported value for basic \c CPUID information is returned.
+///    If \a __leaf == 0x8000000, the highest supported value for extended
+///    \c CPUID information is returned.
+/// \param __sig
+///    If the \a __sig pointer is non-null, the first four bytes of the
+///    signature (as found in the \c ebx register) are returned in the
+///    location pointed to by \a __sig.
+/// \returns Returns 0 if \c CPUID is supported; otherwise returns the value
+///    that \c CPUID returns in the \c eax register.
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phoebewang wrote:

nit: would it better to use upper case `\c EAX`. The same for below.

https://github.com/llvm/llvm-project/pull/170507