win,doc: path encoding in uv_fs_XX is UTF-8

Fixes: https://github.com/libuv/libuv/issues/1554
PR-URL: https://github.com/libuv/libuv/pull/1943
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

docs/src/fs.rst

@@ -12,6 +12,8 @@ otherwise it will be performed asynchronously.
 All file operations are run on the threadpool. See :ref:`threadpool` for information
 on the threadpool size.
 
+.. note::
+     On Windows `uv_fs_*` functions use utf-8 encoding.
 
 Data types
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