Update documents about building Wasm with with-sdk

[lum1n0us]

[skip ci]

Check with grammarly

[lum1n0us]

there may be a chance not to maintain wamr-sdk/app/wamr_toolchain.cmake and wamr-sdk/app/wasi_toolchain.cmake by set different CMAKE_SYSROOT in the command line.

$ cmake -DWASI_SDK_PREFIX=${WASI_SDK_INSTALLTION_DIR}
        -DCMAKE_TOOLCHAIN_FILE=${WASI_SDK_INSTALLTION_DIR}/share/cmake/wasi-sdk.cmake
        -DCMAKE_SYSROOT=${WASI_SDK_INSTALLTION_DIR}/share/wasi-sysroot
        ..

$ cmake -DWASI_SDK_PREFIX=${WASI_SDK_INSTALLTION_DIR}
        -DCMAKE_TOOLCHAIN_FILE=${WASI_SDK_INSTALLTION_DIR}/share/cmake/wasi-sdk.cmake
        -DCMAKE_SYSROOT=${WAMR_SOURCE_ROOT}/wamr-sdk/app/libc-builtin-sysroot
        ..