[clang] [Headers][X86] Add more descriptions to ia32intrin.h and immintrin.h (PR #77686)

[Phoebe Wang via cfe-commits]

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@@ -173,25 +183,59 @@ __popcntq(unsigned long long __A)
 #endif /* __x86_64__ */
 
 #ifdef __x86_64__
+/// Returns the program status and control \c RFLAGS register with the \c VM
+///    and \c RF flags cleared.
+///
+/// \headerfile <x86intrin.h>
+///
+/// This intrinsic corresponds to the \c PUSHFQ + \c POPQ instruction sequence.
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phoebewang wrote:

Also `POP` instead `POPQ`. The same below.

https://github.com/llvm/llvm-project/pull/77686