curl_easy_cleanup: clarify the callback situation

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.\" *

.\" * Copyright (C) 1998 - 2007, Daniel Stenberg, <daniel@haxx.se>, et al.

.\" * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.

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.\" * This software is licensed as described in the file COPYING, which

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has kept open until now. Don't call this function if you intend to transfer

more files.

Any uses of the \fBhandle\fP after this function has been called are

illegal. This kills the handle and all memory associated with it!

Occasionally you may get your progress callback or header callback called from

within \fIcurl_easy_cleanup(3)\fP (if previously set for the handle using

\fIcurl_easy_setopt(3)\fP). Like if libcurl decides to shut down the

connection and the protocol is of a kind that requires a command/response

sequence before disconnect. Examples of such protocols are FTP, POP3 and IMAP.

Any uses of the \fBhandle\fP after this function has been called and have

returned, are illegal. This kills the handle and all memory associated with

it!

With libcurl versions prior to 7.17.: when you've called this, you can safely

remove all the strings you've previously told libcurl to use, as it won't use