mention what happens to the data when a write callback returns pause

A connection can made to pause by using this function or by letting the read

or the write callbacks return the proper magic return code

(\fICURL_READFUNC_PAUSE\fP and \fICURL_WRITEFUNC_PAUSE\fP).

(\fICURL_READFUNC_PAUSE\fP and \fICURL_WRITEFUNC_PAUSE\fP). A write callback

that returns pause signals to the library that it couldn't take care of any

data at all, and that data will then be delivered again to the callback when

the writing is later unpaused.

NOTE: while it may feel tempting, take care and notice that you cannot call

this function from another thread.