[mlir][llvmir] fix docs (#71765)

    The `call` instruction supports both direct and indirect calls. Direct calls

    start with a function name (`@`-prefixed) and indirect calls start with an

    SSA value (`%`-prefixed). The direct callee, if present, is stored as a

    function attribute `callee`. If the callee is a variadic function, then the

    `callee_type` attribute must carry the function type. The trailing type list

    contains the optional indirect callee type and the MLIR function type, which

    differs from the LLVM function type that uses a explicit void type to model

    functions that do not return a value.

    function attribute `callee`. For indirect calls, the callee is of `!llvm.ptr` type

    and is stored as the first value in `callee_operands`. If the callee is a variadic

    function, then the `callee_type` attribute must carry the function type. The

    trailing type list contains the optional indirect callee type and the MLIR

    function type, which differs from the LLVM function type that uses a explicit

    void type to model functions that do not return a value.

    Examples:

    llvm.call @bar(%0) : (f32) -> ()

    // Indirect call with an argument and without a result.

    %1 = llvm.mlir.addressof @foo : !llvm.ptr

    llvm.call %1(%0) : !llvm.ptr, (f32) -> ()

    // Direct variadic call.