NFC: Fix some documentation warnings.

Author: tshortli

include/swift/AST/Decl.h

@@ -1236,12 +1236,12 @@ class alignas(1 << DeclAlignInBits) Decl : public ASTAllocated<Decl> {
 
   /// Returns true if this declaration should be considered available during
   /// SIL/IR lowering. A declaration would not be available during lowering if,
-  /// for example, it is annotated as unavailable with \c @available and
+  /// for example, it is annotated as unavailable with `@available` and
   /// optimization settings require that it be omitted.
   bool isAvailableDuringLowering() const;
 
   /// Returns true if ABI compatibility stubs must be emitted for the given
-  /// declaration. Decls marked unavailable with \c @available require these
+  /// declaration. Decls marked unavailable with `@available` require these
   /// stubs if the compiler flags have enabled unavailable declaration ABI
   /// compatibility mode.
   bool requiresUnavailableDeclABICompatibilityStubs() const;