Add a link to the specification to explain that i is not an extension

Author: lukel97

llvm/lib/Support/RISCVISAInfo.cpp

@@ -470,7 +470,10 @@ std::vector<std::string> RISCVISAInfo::toFeatures(bool AddAllExtensions,
                                                   bool IgnoreUnknown) const {
   std::vector<std::string> Features;
   for (const auto &[ExtName, _] : Exts) {
-    if (ExtName == "i") // i is not recognized in clang -cc1
+    // i is a base instruction set, not an extension (see
+    // https://github.com/riscv/riscv-isa-manual/blob/main/src/naming.adoc#base-integer-isa)
+    // and is not recognized in clang -cc1
+    if (ExtName == "i")
       continue;
     if (IgnoreUnknown && !isSupportedExtension(ExtName))
       continue;