doc: improve uv_fs_readdir() cleanup docs
doc: improve uv_fs_readdir() cleanup docs

When cleaning up after uv_fs_readdir(), it is important to
call uv_fs_req_cleanup() on the readdir request prior to calling
uv_fs_closedir(), because the latter frees memory that the
former needs in order to do its job.

PR-URL: https://github.com/libuv/libuv/pull/2497
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

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