CURLOPT_ACCEPT_ENCODING.3: add brotli and clarify a bit

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.\" * Copyright (C) 1998 - 2016, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2018, Daniel Stenberg, <daniel@haxx.se>, et al.

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Sets the contents of the Accept-Encoding: header sent in an HTTP request, and

enables decoding of a response when a Content-Encoding: header is received.

Three encodings are supported: \fIidentity\fP, meaning non-compressed,

\fIdeflate\fP which requests the server to compress its response using the

zlib algorithm, and \fIgzip\fP which requests the gzip algorithm.

If a zero-length string is set like "", then an Accept-Encoding: header

containing all built-in supported encodings is sent.

libcurl potentially supports several different compressed encodings depending

on what support that has been built-in.

To aid applications not having to bother about what specific algorithms this

particular libcurl build supports, libcurl allows a zero-length string to be

set ("") to ask for an Accept-Encoding: header to be used that contains all

built-in supported encodings.

Alternatively, you can specify exactly the encoding or list of encodings you

want in the response. Four encodings are supported: \fIidentity\fP, meaning

non-compressed, \fIdeflate\fP which requests the server to compress its

response using the zlib algorithm, \fIgzip\fP which requests the gzip

algorithm and (since curl 7.57.0) \fIbr\fP which is brotli.

Set this option to NULL to explicitly disable it, which makes libcurl not send

an Accept-Encoding: header and not decompress contents automatically.